Applied Science, Faculty of
Electrical and Computer Engineering, Department of
DSpace
UBCV
Preston, Garth Eden Claire
Master of Applied Science - MASc
University of British Columbia
In soft electronics technology, stretchable displays move away from conventional rigid devices to adopt mechanical properties more similar to organic materials while preserving function, leading to more conformable and biointegrable devices. Progress has been made on luminescent stretchable displays; however, electrochromic materials that change colour instead of emitting light provide key advantages of low power operation, stability and high contrast. Despite this, development of stretchable electrochromic displays has been limited. In this thesis we demonstrate the first intrinsically stretchable fully integrated electrochromic display device. A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conducting polymer and (1-butyl-3-methylimidazolium octyl sulfate) ionic liquid conductive composite is used as the intrinsically stretchable electrochromic electrodes, with a stretchable polyvinyl alcohol and phosphoric acid-based electrolyte and transparent styrene-based elastomer substrate to form a single pixel device. Both a transparent and reflective opaque version of the device are made. High contrasts up to ΔTmax=15% for transparent and ΔRmax=20% for opaque cells are shown, and optical modulation degradation as low as ΔT or ΔR=1-3% is shown when the samples are strained from 0% to 30% length. Under 30% strain they show fast switch times <7 s, high contrast for up to 500 electrochemical cycles, and low power operation. They perform comparably to other PEDOT based flexible electrochromics with the added benefit of stretchability. A 3×3 passive matrix display is fabricated and high contrast low cross talk single pixel activation addressability is demonstrated under 0% and 30% strain, the first intrinsically stretchable multi-pixel electrochromic display. Fabrication is performed using solution-processing techniques with low cost materials, enabling easy potential scale up. An optical and electrochemical characterization setup is developed to measure performance under strain. An electrochemical technique is newly demonstrated to investigate half cell potentials in the solid-state electrochromic cell as a method for investigating degradation. Overall, an advance is demonstrated in the direction of intrinsically stretchable displays based on electrochromics. Applications include compliant, low cost, visual display interfaces for conveying information, such as physiological monitoring patches, wearable electronics, camouflage coverings, multifunctional tactile platforms, or robotic skin.
INTRINSICALLY STRETCHABLE POLYMER BASED ELECTROCHROMIC DEVICES FOR SOFT ELECTRONIC DISPLAYS by Garth Eden Claire Preston B.Sc., McMaster University, 2017 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Electrical and Computer Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2020 © Garth Eden Claire Preston, 2020 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Intrinsically stretchable polymer based electrochromic devices for soft electronic displays submitted by Garth Eden Claire Preston in partial fulfillment of the requirements for the degree of Master of Applied Science in Electrical and Computer Engineering Examining Committee: John Madden, Electrical and Computer Engineering Supervisor Peyman Servati, Electrical and Computer Engineering Supervisory Committee Member Lorne Whitehead, Physics and Astronomy Supervisory Committee Member iii Abstract In soft electronics technology, stretchable displays move away from conventional rigid devices to adopt mechanical properties more similar to organic materials while preserving function, leading to more conformable and biointegrable devices. Progress has been made on luminescent stretchable displays; however, electrochromic materials that change colour instead of emitting light provide key advantages of low power operation, stability and high contrast. Despite this, development of stretchable electrochromic displays has been limited. In this thesis we demonstrate the first intrinsically stretchable fully integrated electrochromic display device. A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conducting polymer and (1-butyl-3-methylimidazolium octyl sulfate) ionic liquid conductive composite is used as the intrinsically stretchable electrochromic electrodes, with a stretchable polyvinyl alcohol and phosphoric acid-based electrolyte and transparent styrene-based elastomer substrate to form a single pixel device. Both a transparent and reflective opaque version of the device are made. High contrasts up to ΔTmax=15% for transparent and ΔRmax=20% for opaque cells are shown, and optical modulation degradation as low as ΔT or ΔR=1-3% is shown when the samples are strained from 0% to 30% length. Under 30% strain they show fast switch times <7 s, high contrast for up to 500 electrochemical cycles, and low power operation. They perform comparably to other PEDOT based flexible electrochromics with the added benefit of stretchability. A 3×3 passive matrix display is fabricated and high contrast low cross talk single pixel activation addressability is demonstrated under 0% and 30% strain, the first intrinsically stretchable multi-pixel electrochromic display. Fabrication is performed using solution-processing techniques with low cost materials, enabling easy potential scale up. An optical and electrochemical characterization setup is developed to measure performance under strain. An electrochemical technique is newly demonstrated to investigate half cell potentials in the solid-state electrochromic cell as a method for investigating degradation. Overall, an advance is demonstrated in the direction of intrinsically stretchable displays based on electrochromics. Applications include compliant, low cost, visual display interfaces for conveying information, such as physiological monitoring patches, wearable electronics, camouflage coverings, multifunctional tactile platforms, or robotic skin. iv Lay Summary This thesis presents the development and characterization of a novel stretchable display based on polymers that are conductive and change colour under low power electrical operation. The goal was to implement recently reported highly stretchable polymer films in stretchable display devices based on perceived contrast from reflection rather than light emission. First, stretchable colour-changing and conductive polymer films were characterized defining visual, electronic and strain capabilities. Next, integration with additional required stretchable device layers was implemented to develop the final display device. Its visual contrast, speed, electronic behaviour and operation lifetime were investigated, and a stretchable multi-pixel display was demonstrated. This is the first full display device based on stretchable versions of these colour changing materials that we know of. There is wide applicability to conformable visual display interfaces for soft electronics for biomedical, wearable or robotic applications. v Preface The author is the main researcher in work in this thesis, done under the supervision of Dr. John D. W. Madden who provided guidance, technical and editorial feedback. The author proposed this research project under the supervisor’s novel electronic materials research program and has performed the vast majority of the experimental and device design, research data collection, and analysis. Material presented in Chapter 2 has been presented at the EuroEAP 2018, 8th International Conference on Electromechanically Active Polymer (EAP) transducers and artificial muscles, as poster 2.4.9 titled “Stretchable electrochromic polymer films for soft displays”. Juezhen Zhong, an undergraduate summer researcher, aided in fabrication and characterization of some required materials under the author’s direction. The design, development, analysis, and most of the fabrication and characterization were conducted by the author. Relevant material has also been presented at the NSERC NanoMat CREATE Summer Symposium in August 2018. Material relevant to Chapter 3 was mainly fabricated, processed and characterized by the author. Dr. Cédric Plesse and Dr. Kätlin Rohtlaid, both of the University of Cergy-Pontoise, assisted in fabrication methods, provided some materials as well as characterization advisement, and Dr. Xavier Sallenave, also of the University of Cergy-Pontoise, synthesized Magenta polymer. Juezhen Zhong assisted with fabrication. For Chapter 4, PhD candidate Yuta Dobashi advised on the electrochemical setup and materials, Dr. Mirza Saquib us Sarwar advised on materials and device architecture, Dr. Ngoc Tan Nguyen advised on materials, mechanical setup and electrochemical characterization, and Dr. Daniel Jun advised on optical characterization. Additional electrochemical and optical characterization advice was provided by Dr. Plesse, Dr. Pierre-Henri Aubert, Dr. Giao T. M. Nguyen, Dr. Frédéric Vidal, and Dr. Xavier Sallenave of the University of Cergy-Pontoise. All fabrication, setup design, calibration, and characterization was carried out by the author. For Chapter 5, PhD candidate Yuta Dobashi, Dr. Ngoc Tan Nguyen and Dr. Mirza Saquib us Sarwar advised on materials, device architecture and experimental setup. The strain setup was designed in part by Dr. Ngoc Tan Nguyen. All fabrication, setup design, calibration and characterization was carried out by the author. Material relevant to Chapter 4 and Chapter 5 is being prepared as a publication for a journal submission. vi Table of Contents Abstract .......................................................................................................................................... iii Lay Summary ................................................................................................................................. iv Preface............................................................................................................................................. v Table of Contents ........................................................................................................................... vi List of Tables .................................................................................................................................. ix List of Figures ................................................................................................................................. x List of Abbreviations.................................................................................................................... xvi Acknowledgements .................................................................................................................... xviii Dedication .................................................................................................................................... xix Chapter 1. Introduction ............................................................................................................. 1 1.1 Background ............................................................................................................. 3 1.1.1 Display technologies and developments into deformable displays ............. 3 1.1.2 PEDOT ...................................................................................................... 13 1.1.3 Electrochromic cell structure .................................................................... 22 1.2 Overview ............................................................................................................... 25 Chapter 2. Stretchable electrochromic film ............................................................................ 27 2.1 Background ........................................................................................................... 27 2.1.1 Stretchable and highly conductive doped PEDOT:PSS ............................ 27 2.1.2 Stretchable PEDOT and elastomer composites ......................................... 34 2.2 Fabrication ............................................................................................................ 35 2.2.1 Materials .................................................................................................... 35 2.2.2 PEDOT:PSS DMSO composites ............................................................... 35 2.2.3 PEDOT:PSS ionic dopant composites ...................................................... 36 2.2.4 PEDOT elastomer composites .................................................................. 37 2.3 Characterization .................................................................................................... 37 2.3.1 Morphology ............................................................................................... 37 2.3.2 Electrical.................................................................................................... 37 2.3.3 Mechanical ................................................................................................ 38 2.3.4 Electrochemical ......................................................................................... 39 2.3.5 Optical ....................................................................................................... 39 vii 2.4 Results ................................................................................................................... 39 2.4.1 Morphology ............................................................................................... 39 2.4.2 Electrical.................................................................................................... 41 2.4.3 Mechanical ................................................................................................ 43 2.4.4 Electrochemical ......................................................................................... 45 2.4.5 Optical ....................................................................................................... 48 2.5 Discussion ............................................................................................................. 49 Chapter 3. Stretchable electrolyte ........................................................................................... 50 3.1 Background ........................................................................................................... 50 3.1.1 Electrolytes in electrochromics ................................................................. 50 3.1.2 Polymer electrolytes for electrochromics .................................................. 51 3.1.3 Stretchable gel electrolytes........................................................................ 52 3.1.4 Polyvinyl alcohol....................................................................................... 53 3.2 Fabrication ............................................................................................................ 54 3.2.1 Materials .................................................................................................... 54 3.2.2 Semi-IPN ................................................................................................... 54 3.2.3 PVA/H3PO4 ................................................................................................ 55 3.3 Characterization .................................................................................................... 55 3.3.1 Mechanical ................................................................................................ 55 3.3.2 Electrical.................................................................................................... 56 3.3.3 Electrochemical ......................................................................................... 56 3.3.4 Optical ....................................................................................................... 57 3.4 Results ................................................................................................................... 57 3.4.1 Semi-IPN ................................................................................................... 57 3.4.2 PVA/H3PO4 ................................................................................................ 63 3.5 Discussion ............................................................................................................. 67 Chapter 4. Intrinsically stretchable electrochromic cell ......................................................... 68 4.1 Background ........................................................................................................... 68 4.1.1 Base structure ............................................................................................ 68 4.1.2 Performance improvement ........................................................................ 71 4.1.3 Degradation ............................................................................................... 73 4.1.4 Spectrometry setup background ................................................................ 77 4.2 Fabrication ............................................................................................................ 81 viii 4.2.1 Materials .................................................................................................... 81 4.2.2 Cell construction ....................................................................................... 82 4.3 Characterization .................................................................................................... 85 4.3.1 Optical characterization setups.................................................................. 85 4.3.2 Electrochemical control/characterization .................................................. 89 4.4 Results ................................................................................................................... 92 4.4.1 Fiber optic spectrometer transmission and reflectance comparison.......... 92 4.4.2 Contact design ........................................................................................... 97 4.4.3 Electrochemical ......................................................................................... 99 4.5 Discussion ............................................................................................................117 Chapter 5. Passive matrix display..........................................................................................119 5.1 Background ..........................................................................................................119 5.1.1 Direct drive ...............................................................................................119 5.1.2 Passive matrix ......................................................................................... 120 5.1.3 Active matrix ........................................................................................... 122 5.2 Fabrication .......................................................................................................... 123 5.2.1 Matrix ...................................................................................................... 123 5.2.2 Control matrix PCB ................................................................................. 125 5.3 Characterization .................................................................................................. 125 5.4 Results ................................................................................................................. 126 5.4.1 Electrochemical impedance spectroscopy ............................................... 126 5.4.2 Matrix addressing control........................................................................ 127 5.4.3 Performance under 30% strain ................................................................ 131 5.5 Discussion ........................................................................................................... 133 Chapter 6. Conclusion .......................................................................................................... 136 References ................................................................................................................................... 141 Appendices .................................................................................................................................. 157 Magenta elastomer composite and electrochemical tests .................................. 157 Electrode optimization ...................................................................................... 160 Custom optical spectrometer reference and uncertainties ................................. 162 Cell three-electrode measurements ................................................................... 164 ix List of Tables Table 1: Most advanced towards intrinsically stretchable displays demonstrated with each technology ....................................................................................................................... 12 Table 2: Highest PEDOT:PSS conductivity adding dopants as of 2015, adapted from [52] ........ 29 Table 3: Two-point film resistance measurements and approximate fracture strain ..................... 41 Table 4: Four-point film conductivity measurements ................................................................... 42 Table 5: Step tests for semi-IPN electrolyte cell results, ITO counter electrode (BMIM used to abbreviate for BMIM OSU)............................................................................................ 61 Table 6: Cells using BMIM OSU/PEDOT:PSS as both electrodes .............................................. 62 Table 7: Electrolyte results for PVA/H3PO4 with BMIM OSU/PEDOT:PSS sample .................. 66 Table 8: Remaining uncertainties in the transmission and reflectance setups .............................. 92 Table 9: Best performance quantitatively tested samples (reflectance time constants for faster sample) .......................................................................................................................... 104 x List of Figures Figure 1: Stretchable emissive displays a) phosphor electroluminescent [16], b) rigid LEDs with stretchable interconnects [17], c) polymer OLED [19], d) quantum dot [26], e) light emitting electrochemical cell [31], f) perovskite [34], reproduced with permission ........ 5 Figure 2: Flexible and stretchable electrochromic devices: a) flexible active matrix PEDOT display [10], b) [11] flexible PEDOT electrochromic display, c) stretchable WO3 electrochromic device [48] , d) electrochromic spandex in electrolyte [50] , e) intrinsically stretchable PEDOT/PU [5] reproduced with permission ............................11 Figure 3: Examples of thiophene-derived electrochromic films [46] reproduced with permission .........................................................................................................................................11 Figure 4: a) PEDOT structure, b) conjugated backbone, c) red sp2 hybridized orbitals in sigma bonds and blue π orbitals out of plane, d) delocalization of π orbitals along backbone, e) effect of oxidative doping with reference to [51] ........................................................... 14 Figure 5: Change of electronic band structure in conducting polymers as they are further oxidatively doped until behaving like metals, adapted from [52] .................................. 16 Figure 6: PEDOT:PSS optical absorption spectra for different applied voltages in electrochemical cell, permission from [51] original from [54] reproduced with permission. Visible light is in the range of 1.8-3.1 eV. ................................................... 17 Figure 7: a) PEDOT (blue):PSS (yellow) coil structure in water dispersion, b) proposed molecular structure adapted from [52] ........................................................................... 18 Figure 8: Acid catalyzed dimer and trimer formation beginning PEDOT chain adapted from [51] ........................................................................................................................................ 19 Figure 9: Cyclic voltammetry a) example of triangular potential scan, b) resulting voltammogram for reversible redox couple (blue) and capacitive structure (red), c) cyclic voltammograms of PEDOT:PSS films in different electrolytes, reproduced with permission from [61] ...................................................................................................... 21 Figure 10: Typical electrochromic cell structure .......................................................................... 23 Figure 11: Alternation between benzoid and more linear quinoid structure, adapted from [52] .. 28 Figure 12: Phase images of PEDOT:PSS with dopants a) DMSO b, c, d) conductivity and stretchability enhancers, e) stress strain behaviour, and for STEC1 f) stress/strain xi behaviour, g) relaxation time, h) storage and loss modulus, i, j) theorized nanomorphology change, reproduced with permission from [6] .................................... 30 Figure 13: a) Molecular structure of BMIM OSU, b) dependence of PEDOT:PSS phase on dispersion ionic strength with dopant additive. From [79] with permission .................. 33 Figure 14: As prepared films: a) 46% BMIM OSU/PEDOT:PSS on glass, b) 20%, 30%, 40% PEDOT/PU on glass, c) 46% BMIM OSU/PEDOT:PSS on SEBS, d) 30% PEDOT/PU on PU .............................................................................................................................. 36 Figure 15: Mechanical strain setup for crack onset measurements .............................................. 38 Figure 16: AFM images of PEDOT:PSS composite films with a) 5% DMSO, b) 46% LiTFSI, c) 46% BMIM OSU, phase images scaled to same º range, 1 μm image width ................. 40 Figure 17: AFM height and phase images different ratios of PEDOT to PU in PEDOT/PU composites- height is in m, phase in º ............................................................................. 40 Figure 18: Example film thickness measurement – right: AFM height image 80 µm wide with a profile taken along the red line, left: height profile showing ~600 nm step of PEDOT to glass, x axis in µm across the sample ............................................................................. 42 Figure 19: Example films under strain through optical microscope, left: doped films (irregularities due to impurities or spin coating nonuniformity), showing initial cracks in film ~20% strain, and cracking over entire film at 25% strain, right: PEDOT/PU film on PU strained initially on 5X magnification, then increased to 20X, showing no clear evidence of mechanical degradation up to 82% strain, features include air bubbles between PEDOT/PU layer and PU from bonding .......................................................... 44 Figure 20: a), b) 1 V step response of films on ITO showing time constant and thickness; cyclic voltammograms of c) different PEDOT films at 100 mV/s, d) LiTFSI/PEDOT:PSS .... 46 Figure 21: UV-Vis spectroscopy measurements of films with typical thickness t. left: ΔTmax= 47% (603 nm), middle: 36% (620 nm), right: 34% (555 nm) respectively .................... 48 Figure 22: Photos of electrochromic films in oxidized and reduced states, a) LiTFSI/PEDOT:PSS b) PEDOT/PU ................................................................................................................. 48 Figure 23: Circuit model for electrical impedance spectroscopy measurement, adapted from [113] ................................................................................................................................ 56 Figure 24: a) Nyquist plots AiBN and DCPD initiated electrolytes showing x intercept to calculate ionic conductivity, b) DCPD semi-IPN, c) AiBN semi-IPN ........................... 58 xii Figure 25: Cells in reduced and oxidized states using semi-IPN electrolyte a) (BMIM OSU/PEDOT:PSS)/DCPD semi-IPN/ITO, b) (BMIM OSU/PEDOT:PSS)/AiBN semi-IPN/ITO, c) PEDOT/PU on (BMIM OSU/PEDOT:PSS)/DCPD semi-IPN/ITO, (Reduced: -3 V, Oxidized:+1 V across cell), d) (BMIM OSU/PEDOT:PSS)/DCPD semi-IPN/(BMIM OSU/PEDOT:PSS), e) (BMIM OSU/PEDOT:PSS)/AiBN semi-IPN/(BMIM OSU/PEDOT:PSS), f) PEDOT/PU on (BMIM OSU/PEDOT:PSS)/DCPD AiBN/(BMIM OSU/PEDOT:PSS) (Reduced: -3 V, Oxidized: 0 V across cell) ............. 59 Figure 26: Typical data set for step tests, a) step response, b) Nyquist plot and c) L* contrast ... 60 Figure 27: Magenta/polyurethane (M) cells using DCPD semi-IPN with EMITFSI, a) M on ITO vs. ITO, b) M on BMIM OSU/PEDOT:PSS (B) vs. ITO, c) M on B vs. B .................... 63 Figure 28: PVA/H3PO4 characterization a) stress strain for 1:1.5 PVA:H3PO4, b) transparent and TiO2 opaque versions, stretch to 250%, c) Nyquist plots for ionic conductivity calculation ....................................................................................................................... 63 Figure 29: PVA/H3PO4 electrolyte cells a), b) cells using transparent and opaque electrolyte on different surfaces, c) transparent cell switch, 3V d) later opaque cell switch, 1.6 V ...... 64 Figure 30: Stability window of H3PO4 electrolytes with cyclic voltammetry (10 mV/s): a) three electrode aqueous H3PO4 using carbon black and ITO working electrodes and a carbon paper counter electrode, b) two electrode tests of carbon black/(Solid PVA/H3PO4)/carbon black cells ...................................................................................... 65 Figure 31: a) Step potential on PVA/H3PO4 sample, b) L* contrast from video .......................... 66 Figure 32: a) Spectral responsivities of retinal cone cells, b) RGB colour matching functions, c) XYZ colour matching functions, reproduced with permission [165] ............................. 79 Figure 33: a) Fabrication process full cell, b) opaque sample with pseudo reference Ag/AgCl wire inserted .................................................................................................................... 82 Figure 34: a, b) Transmission fixture components, blue nuts are secured with super glue, c) fixture with fiber optic cables, d) fixture with sample mounted on BOSE clamps, e) full optical transmission experimental setup and light path .................................................. 84 Figure 35: a, b) Reflectance fixture components, blue nuts are secured with super glue, c) fixture with fiber optic cable and reflectance probe, RS50 white standard, d) fixture with sample mounted on BOSE clamps, e) full reflectance experimental setup and light path ........................................................................................................................................ 85 xiii Figure 36: Averaging colour data over sample area and reference white area ............................. 86 Figure 37: Light source standard spectral variations over 9 minutes with 2 layers Colour Temp. Blue filter, 1 ms integration time, average over 100 frames ........................................... 92 Figure 38: a) Measurement of neutral density (ND) filters using the Cary and Comet, and light source spectrum, b) difference of transmission between the Cary and Comet, demonstrating linear relation to light source intensity, c) absolute transmission of sample measured with Cary and Comet, d) ΔT calibration against sample measured with Cary 7000 ........................................................................................................................ 94 Figure 39: a) Cary SCE and Comet measurement, b) ΔR% between Cary and Comet, c) Rcomet-Rcary vs light power ......................................................................................................... 95 Figure 40: a) Transmission of a sample in reduced and oxidized states, b) images of sample in reduced and oxidized states, c) overlay of T and L*/L0* calculated from sRGB video showing approximate correlation ................................................................................... 96 Figure 41: a) Reflectance of a sample in reduced, 0 V and fully oxidized states, b) images of sample in these 3 charge states, c) luminosity of sample from sRGB video with white reference of these three states, d) overlay of R and L*/L0* calculated from sRGB different video when switching -1.6 to +1.6 V ............................................................... 97 Figure 42: Behaviour under strain of different iterations of contacts: a) resistance, b) contrast vs strain, c) contrast with cycles different for samples. ...................................................... 98 Figure 43: a) Cyclic voltammogram to select Vstep, b) I, V and L* with time .............................. 99 Figure 44: Nyquist plots comparing typical transparent and opaque cells for frequency 0.05 Hz-0.8 MHz ........................................................................................................................ 100 Figure 45: Transparent sample a) visual contrast with strain, b) L* video values with strain .... 101 Figure 46: Comparing single cell results under 0% and 30% strain: a) Transmission, b) ΔT, c) ΔTmax and current, d) reflectance, e) ΔR, f) ΔRmax and current ................................... 102 Figure 47: Opaque sample a) visual contrast at different strains, b) L* contrast with strain ..... 103 Figure 48: Impedance of a transparent cell: effects of strain and electrochemical cycling. Left: impedance at strains between 0% and 30%, before and after electrochemical cycles, Right: close-up around real axis showing effect on internal resistance of strain and cycling ........................................................................................................................... 106 xiv Figure 49: Transparent sample cycle performance under 30% strain, a) contrast at different cycle numbers, b) contrast over ~90 cycles, c) impedance spectroscopy, d) images ............. 106 Figure 50: Example cycling performance of opaque sample under 30% strain, a) contrast at different cycle numbers, b) contrast over ~60 cycles, c) impedance spectroscopy, d) images ........................................................................................................................... 107 Figure 51: Cycle performance of opaque sample 500 cycles, a) contrast at different cycles, b) first 10 cycles, c) EIS plot, d) photos at different cycles, e, f) show original and vertically shifted contrasts over 500 cycles adjusted so oxidized L*=0, with different colours corresponding to sequential video files ............................................................ 108 Figure 52: Cyclic voltammetry comparing ps-Ag/AgCl with standard Ag/AgCl, tested in a) aqueous H3PO4, b) solid PVA/H3PO4 ............................................................................110 Figure 53: a) Cyclic voltammogram 2-electrode (10 mV/s), b), c) 3-electrode measurements voltage division across transparent and opaque half-cells vs.ps-Ag/AgCl, (bottom electrode potential is negative, shown flipped in polarity to illustrate difference from top voltage) ..........................................................................................................................112 Figure 54: a) Cyclic voltammogram three electrode measurements (10 mV/s), b), c) 3 electrode voltage division and optical response across a transparent (b) and an opaque (c) cell, plotting absolute value of voltage vs. ps-Ag/AgCl, top (black) is reducing first ..........115 Figure 55: a) Direct drive 7 segment display, b) passive matrix architecture V/2 protocol, adapted from [12] c) V/3 protocol adapted from [9] ...................................................................119 Figure 56: Passive matrix pixel voltage threshold requirement, reproduced from [12] with permission ..................................................................................................................... 120 Figure 57: IV behaviour of a) a symmetrical electrochromic PEDOT:PSS pixel without threshold voltage b) a pixel exhibiting threshold voltage using PEDOT:PSS and carbon paste as electrodes, c) printed passive matrix display, reproduced from [9] with permission ... 121 Figure 58: Matrix fabrication procedure steps shown ................................................................ 123 Figure 59: a, b) Demonstrated flexibility of matrix prior to electrical control testing, c) PCB for digital to analog 4×4 device channel control ................................................................ 124 Figure 60: Nyquist plots for electrical impedance spectroscopy. The real (resistive) and imaginary (capacitive) impedance of each pixel are compared and shown to be similar, xv denoted by ‘R’ for row and ‘C’ for column number. Inset shows intersection of x axis for all indicating cell resistance ~250 Ω for all pixels. ................................................. 126 Figure 61: Activation of individual pixels using constant partial voltage addressing V/3 protocol under a) 0% strain, and b) 30% strain. We observe that single pixels activated in order 1-9 demonstrate single pixel addressability both in their undeformed and deformed state, demonstrating our final intrinsically stretchable multi-pixel electrochromic display. . 129 Figure 62: Luminosity L* and Change in Luminosity under 0% and 30% strain for each pixel 1-9 ...................................................................................................................................... 130 Figure 63: Magenta/PU on a) ITO, b) 46% BMIMOSU/PEDOT:PSS, c) PDMS-coated hydrophobic glass, d) after UV-Ozone treatment and PU thermal + pressure transfer 157 Figure 64: Magenta/PU composite optical microscope, and strain to 100% then return to 0% . 157 Figure 65: Step response different magenta cells, showing voltage applied across cell (a-c), L*a* b* values unnormalized (d-f), and electrochemical impedance plots (g-i) for: a, d, g) magenta on ITO vs. ITO, b, e, h) magenta on BMIM OSU/PEDOT:PSS (B) vs. ITO, c, f, i) magenta on B vs. B counter electrode ....................................................................... 159 Figure 66: Mechanical failure of contact .................................................................................... 160 Figure 67: Early iterations of contact fabrication ....................................................................... 160 Figure 68: Cary 7000 specular component included vs excluded (SCI vs SCE) and dR (%) comparison .................................................................................................................... 162 Figure 69: a) Mounting of sample with mask onto DRA 2500 external reflectance attachment for Cary 7000, baseline 100% and 0% reflectance for without b) and with c) small sample mask .............................................................................................................................. 162 Figure 70: Variation between runs for sample transmission Cary 7000 (~2.7%) ....................... 163 Figure 71: Cyclic voltammograms (10 mV/s) of standard Ag/AgCl and pseudoreference Ag/AgCl using hydroquinone redox couple in aqueous H3PO4 ................................... 164 Figure 72: Step voltage division across cell, with half-cell voltages vs. pseudoreference Ag/AgCl wires, and luminosity L* .............................................................................................. 164 xvi List of Abbreviations AC – alternating current AFM – atomic force microscopy AM – active matrix BMIM OSU – 1-butyl-3-methylimidazolium octyl sulfate CB – carbon black CIE – International Commission on Illumination D65 – daylight 6500 K illuminant standard DAC – digital to analog converter DMA – dynamic mechanical analyzer DMSO – dimethylsulfoxide EDOT – 3,4-ethylenedioxythiophene EIS – electrochemical/electrical impedance spectroscopy EMITFSI – 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IPN – interpenetrating network iTMC – inorganic transition metal complex ITO – indium tin oxide LCD – liquid crystal display LED – light emitting diode MOSFET – metallorganic semiconductor field effect transistor ND – neutral density NW – nanowires OCP – open circuit potential OLED – organic light emitting diode PCB – printed circuit board PEDOT – poly(3,4-ethylenedioxythiophene) PMMA – poly(methyl methacrylate) PSS – poly(styrenesulfonate) PU – polyurethane PVA – polyvinyl alcohol PVDF – polyvinylidene fluoride xvii QD – quantum dot RGB – Red Green Blue colour space SCE/SCI – specular component excluded/included SEBS – styrene-ethylene-butadiene-styrene block copolymer SHE – standard hydrogen electrode SIS – styrene-isoprene-styrene block copolymer TFEL – thin film electroluminescence THF – tetrahydrofuran UV – ultraviolet xviii Acknowledgements Thank you to my supervisor Dr. John Madden. Thank you to all those who contributed to the completion of this project in time, materials, equipment or other support including but not limited to, at UBC: especially Dr. Mirza Saquib us Sarwar, Yuta Dobashi, Dr. Ngoc Tan Nguyen, Dr. Daniel Jun and Juezhen Zhong, as well as Dr. Saeedeh Ebrahimi Takalloo, Dr. Thomas Beatty, and Dr. Konrad Walus. At the University of Cergy Pontoise: Dr. Cédric Plesse, Dr. Kätlin Rohtlaid, Dr. Frédéric Vidal, Dr. Giao T. M. Nguyen, Dr. Xavier Sallenave, and Dr. Pierre-Henri Aubert. Thank you also to Caelin Preston, Garth Preston, and Mei Mei Wu-Preston for their everlasting support, as well as the rest of my wonderful family. xix Dedication Dedicated to my parents, my brother, and the rest of the family. Special mention for Mer Mer for teaching me to create and innovate from a young age. 1 Chapter 1. Introduction As we move into an age where electronics are becoming more integrated into our lives, there is growing interest in more adaptable biointegrable and bio-inspired systems. Flexible and stretchable electronics have begun to pervade by integrating novel materials and structures into devices to impart conformability previously limited by rigidity [1]–[3]. There is vast applicability to expandable and foldable touch screens, wearable technology, physiological monitoring, camouflage and soft robotic skin. As we adapt our devices to better take on the deformable properties, we approach more seamless and comfortable integration with our bodies and a variety of surfaces. The major interest in the field is to produce electronics that can be deformed while still maintaining similar performance to rigid devices. Working towards improving silicon-based hard devices by emphasizing speed and efficiency is drastically different from working towards deformability. The latter requires focusing on features such as durability, light weight, thinness, and conformability, while preserving required functional electrical performance. Work in this area began only in the 1990’s, with the emergence of two main approaches: 1. stretchable structures connecting rigid devices, or 2. materials that can themselves stretch-intrinsic stretchability [1]. Some approaches to the first include winding metallic interconnects connecting rigid LEDs or other devices embedded in a stretchable substrate, or creating buckling structures of conductive material to allow some strain before breakage [1]. Approaches to the latter include mainly composites, either mixing electrically active materials such as nanomaterials into elastomers or employing stretchable organic polymeric materials [2], [4] . Most efforts are focused on thin, lightweight electrodes towards biointegrable, wearable applications, focusing on strengths such as low power, mechanical durability, and ambient stability. Sensors, optoelectronics and transistor arrays are only a few examples of recent developments. This work focuses on development of stretchable low power reflective displays based on intrinsically stretchable electrochromic and conducting polymer materials as an advantageous approach to integrating stretchability into electronic visual interfaces. I began this project in deciding to investigate whether or not a stretchable electrochromic composite could be used to make a fully integrated intrinsically stretchable electrochromic display device, which would be the first of its kind. This would require taking the conventional 2 layers needed for an electrochromic device, currently commercially at the flexible device stage – transparent electrodes, active electrochromic layer, ionically conducting separator, ion storage layer – finding stretchable electrochemically compatible materials for each and building the structure such that it can sustain performance under strain. First, we planned to investigate if we could fabricate previous poly(3,4-ethylenedioxythiophene) (PEDOT)-based stretchable films and test them for electrochromic capability in addition to verifying their mechanical properties. We theorized this should be possible as there were previous reports of many PEDOT-based flexible electrochromic devices and several intrinsically stretchable conductive PEDOT films, one that had also been shown to be electrochromic [5], [6]. Then we sought to investigate whether this material itself could be used as a stretchable electrode, replacing brittle conventional indium tin oxide (ITO) transparent electrodes in flexible electrochromic devices to enable strain. Based on recently demonstrated PEDOT stretchable electrodes, we theorized that it could be possible [6], [7]. We also predicted it could be used as the ion storage layer, as in previous electrochromic devices [8]. Usage for all these layers would enable a simplified device structure. Once we had identified a stretchable electrochromic film, we then planned to investigate whether previously reported stretchable highly ionically conductive electrolytes could be integrated to form an electrochemically and mechanically compatible interface with the PEDOT, electrical contacts, and encapsulating substrate to form a full intrinsically stretchable electrochromic vertically layered device. We thought this should be possible as there are several stretchable polymer electrolytes with high (~10-3 S/cm) ionic conductivity but were unsure whether they could withstand strain while also preserving the electrochemical interface with PEDOT. If we accomplished this, we then wanted to quantify the performance change with strain, and compare it to other non-stretchable devices in the field in terms of contrast, speed, and lifetime. As our last step in future directions we wanted to see if we could form an addressable multi-pixel display moving towards more complex displays for conveying information. It was believed that this should be possible based on previous developments in matrix electrochromic flexible displays, with the only additional aspect being stretchable materials [9]–[11]. In order to motivate this ideal approach among other display technologies, first an overview of the most prominent display technologies will be given and their developments into flexible displays, as well as promise for stretchable displays. Merits of electrochromism and conductive 3 electrochromic materials for stretchable displays will be touched on and basics of their properties will be explained. In our experimental chapters we will go over the steps to develop and test our device: the stretchable electrochromic film, the stretchable electrolyte, the integrated cell fabrication and testing, and the stretchable matrix. Overall, the devices we conclude with successfully achieve our goals: 1. a single electrochromic pixel that shows intrinsic stretchability with stable performance at 30% strain similar to its unstrained state, and 2. a multi-pixel passive matrix display that has singly addressable pixels that also operates under 30% strain. Our final device is composed of polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) in a composite with 1-butyl-3-methylimidazolium octyl sulfate (BMIM OSU) ionic liquid as both the electrochromic and electrode material, a polyvinyl alcohol (PVA) based electrolyte separator, a supporting encapsulating elastomer substrate, and soft carbon-based interconnects. For novelty: as far as we know this is the first intrinsically stretchable fully integrated electrochromic device based on an intrinsically stretchable electrochromic film and the first multi-pixel array of this form as well. Previously intrinsically stretchable electrochromic films have been demonstrated but not a fully integrated device that remains stretchable. It is also the first intrinsically stretchable electrochromic device to be all-polymer, not requiring nanomaterial or metal electrodes for operation, and solid state. 1.1 Background 1.1.1 Display technologies and developments into deformable displays Commercial display technologies developed from cathode-ray-tube devices into flat panel devices. Below we give an overview of main display types and challenges in making them deformable and stretchable to set a framework for where our work fits in. These will cover emissive displays that have had significant commercial or research developmental success: phosphors, LED, OLED, LCD, and non-emissive reflective displays, with a focus on the material requirements and limitations. Conventional display technologies have almost all been developed around a rigid architecture and components. In exploring their transition from rigid to flexible and soft displays, many exhibit shortcomings limited by the materials and architectures in conventional electronics. Many adaptations rely on structurally stretchable devices, employing rigid emissive lights connected by winding or buckling interconnects that are deformable, 4 limiting resolution and strain depending on degree of substrate prestrain, while some others employ inherently stretchable materials. 1.1.1.1 Phosphor-based electroluminescence The first commercial displays were based on phosphors, solid materials that luminesce after absorbing energy from an external source. This forms the basis of cathode ray tube, vacuum fluorescent and plasma displays, where electrons hit a phosphor-coated transparent surface causing light emission. These displays are high speed and very successful but require large volumes to accelerate electrons. Phosphors also form the basis of thin film electroluminescent display (TFEL) research based on alternating current electric field applied to powder phosphors to generate electroluminescence, first developed in the 1950s [12]. This has been used to produce deformable devices based on high voltage and frequency alternating current electroluminescence. Many composites containing the active material can be made intrinsically stretchable by embedding the phosphors into an elastomer layer. Devices using ZnS:Cu phosphors operated at 100-1000 V, 2 kHz demonstrated operation under 700% strain using ionic conductors [13], [14]. Other multicolor ones have been demonstrated for applications to robotic skin shown in Figure 1a [15], [16]. This field of research shows many challenges such as poor stability and brightness often with <500 h device lifetime and high sensitivity to ambient conditions. The materials often degrade because of the high electric field required, but some thin film, high luminance and moderate lifetime devices have been demonstrated, but not yet commercialized [13]. 1.1.1.2 LED displays The development of light emitting diodes arose from the semiconductor industry in the 1960s. These involve a crystalline material that emits light when voltage is applied due to mobile charges jumping across the material’s bandgap [12]. As the blue LED was not developed until the 1990s and miniaturizing and integrating different colour LEDs for high resolution screens is challenging, micro-LED display development is still in its infancy. Because LEDs require crystalline structures in order to perform efficiently, they are only able to be integrated in rigid island, stretchable interconnect structures. An elastomer sheet containing a matrix of rigid 5 AlGaInP micro LEDs joined by winding metal interconnects was reported, allowing 48% strain operation conditions as seen in Figure 1b [17]. 1.1.1.3 LCD displays LCD displays are widely used today in computers, TVs, cell phones, watches, and calculators. The particles in the material orient into crystal-like organized structures when an electric field is applied, changing the optical properties [12]. A common twisted nematic cell found in an LCD display is composed of an LED backlight, colour filter, and two transparent electrodes, one with a polarizer and one with an analyzer that together only transmit certain light polarizations, sandwiching liquid crystal material. When an electric field is applied, the molecules form a twisted structure and the incident light’s polarization is rotated such that it is transmitted. Without electric field and molecular alignment no transmission occurs, such that the pixel appears black. This method can produce a pixel response time <10 ms depending on electrode geometry, but often perception depends on angle of view due to the polarizing layers. The standard requirement is >100 Cd/m2 brightness for a readable display in daylight conditions [12]. A large limitation on LCDs becoming flexible is the backlight power requirement, with an example 42” HDTV requiring 42 mW/cm2. The backlight needs brightness >1500 Cd/m2, as the filtering and polarizing layers block up to 70% of the total light Figure 1: Stretchable emissive displays a) phosphor electroluminescent [16], b) rigid LEDs with stretchable interconnects [17], c) polymer OLED [19], d) quantum dot [26], e) light emitting electrochemical cell [31], f) perovskite [34], reproduced with permission 6 power. Varied performance with viewing angle offers challenges for uniform appearance for flexible devices. Any displays using conventional LCD technology are difficult to make stretchable due to the requirement of all the electrode and polarizing layers that all must stretch while maintaining the optical modulation requirements, the containment of the liquid crystal in a stretchable cell, and the backlight requirement, so little work has been done in this area. 1.1.1.4 Organic light emitting diode Organic light emitting diode (OLED) displays are a major advance arising since the 1980s allowing drastic reduction in required display thickness, larger contrast and higher efficiency. Typically they consist of an organic phosphorescent-doped material based on small molecules or polymers that exhibit semiconducting or metallic properties [12]. While in a typical semiconductor there are separate electron and hole charge carriers, in a typical organic semiconductor, bound states of electrons and holes remain confined to local molecular sites and migrate by hopping between molecules. Devices usually need an anode, hole transport layer, electron transport layer, and a cathode. The electrons and holes meet and recombine, releasing light, with colour depending on the material bandgaps. Compared to the popular LCD displays, OLED displays have much higher efficiency, lower power consumption, and drive current. Because of OLED thin film structure they have emerged as the commercial ideal choice for commercial flexible video displays with several in production, requiring only the organic layers and transparent electrodes [12]. Material-wise, flexible transparent conductor indium tin oxide (ITO) is needed to provide charge flow. However, any substrate for OLEDs must provide high impermeability encapsulation due to active material’s extreme sensitivity to moisture and oxygen, requiring hermetic sealing [18]. In moving towards more conformable versions for wearables and electronic skin, key groups including Someya’s at the University of Tokyo and others have demonstrated several ultraflexible polymer OLEDs using ITO, thin metal or PEDOT:PSS electrodes on polymer substrates that exhibit 29 h half life and high brightness [19]–[21]. Stretchable versions have only shown promise with prestrained substrate structural stretchability techniques, including a device that operates at up to 200% strain shown in Figure 1c. Moving to more elastomeric substrates would provide too poor a barrier against oxygen and moisture due to their increased porosity, and even in the current case the lifetimes are too low for commercializable stretchable 7 devices. This, together with lack of stretchable active materials has prevented development of intrinsically stretchable OLEDs so far. 1.1.1.5 Quantum dot displays Industry and researchers have recently delved into quantum dot (QD) photoluminescent and electroluminescent properties to enhance displays over traditional LCD screens. Quantum dots are semiconductor nanocrystals that exhibit quantum confinement allowing them to emit very tunable narrow wavelengths of colour with optical efficiency higher than any previous luminescing technologies [22]. Commercially they have already been implemented in photo-emissive LCD display devices where solely blue light is emitted from an LED back plane and it stimulates photoluminescence in red and green quantum dots on a front panel surface. The first stage of photoluminescent QD displays still require LCD architecture for switching, limiting adaptability to flexible and stretchable versions. Soon we may see commercial implementation of electroluminescent quantum dot displays – these allow direct luminescence from the quantum dots with electrical signal application, reducing needed power and components even further to solely electrodes and the QD materials. They have been developed in research in 2011 with high quantum efficiency but lifetime still has to be improved to the commercial standard of 10 000 hours [23], [24]. In 2014 a highly flexible quantum dot QLED electro emissive display was demonstrated, operating at a low voltage with no ITO needed [25]. In 2015 a wearable RGB quantum dot array was demonstrated, with 20% strain operation achievable using a prestrained substrate shown in Figure 1d [26]. Despite achieving the best electroluminescent performance of LEDs to date, quantum dots are also highly sensitive to oxidation, with a lifetime of 41 h for the device [26], limiting their application in stretchable devices due to poor encapsulation by elastomeric substrates. 1.1.1.6 Light emitting electrochemical cells (LECs) Light emitting electrochemical cells were first explored in 1990s by Pei and colleagues [27] and have not yet progressed to commercial displays. These are based on light emitting electrochemical reactions using semiconducting polymers or inorganic transition metal complexes (iTMCs). A voltage applied between two ITO electrodes sandwiching a luminescent 8 material causes electrochemical doping, initiating a PN junction. A process equivalent to luminescent recombination occurs where immobile ions are oxidized and reduced and electrons and holes are mobile, and they combine across a bandgap to release a specific wavelength of light. These are low voltage and can have response speeds as low as 5 ms, though they tend to be in the seconds to minutes range [28]. One major challenge is the short lifetime due to junction instability caused by the electrochemical processes. Efforts to stabilize the junction were helpful in enabling passive matrix addressability and high luminous efficiency [29]. Still, stability, colour range limitation, turn on time and lifetime variation issues have prevented increase in popularity for conventional displays [30]. On the other hand, the mechanical properties and requirements for only two electrode layers in addition to the active material, which can be made intrinsically stretchable, make these devices advantageous for stretchable devices. A polymer-based device able to sustain 120% strain during operation with Ag nanowire electrodes was reported in 2013, but with a 10 minute turn on time shown in Figure 1e [31]. A device based on ruthenium dispersed in PDMS able to operate at strain up to 27% and repetitive cycling of 15% strain was also shown [32]. However, these devices are still in the infant stage, with poor performance compared to OLEDs, widely varying turn-on time, and chemical stability problems. 1.1.1.7 Perovskites Organometal-halide-perovskites are best known for research on perovskite based solar cells, but in 2014 the first perovskite-based LED was reported [33]. They have not been developed into displays yet. In 2017 a stretchable version was demonstrated using substrate prestrain and buckling and showed the ability to operate almost at full brightness >1000 cd/m2 at 50% strain shown in Figure 1f [34]. Despite this promising performance, as this is early stage research, instability and lifetime degradation with ambient exposure are still large challenges, with encapsulation development required. 1.1.1.8 Electrophoretic displays So far, we have discussed emissive display technologies, but there are also many non-emissive reflective displays relying on external lighting rather than emission to obtain contrast. Common examples are electrophoretic devices like e-readers and electronic paper. The tradeoff 9 of luminescence offers benefits of high contrast, easy readability in variable conditions, very low power, flexibility and durability [12]. The technology involves manipulating white and black particles suspended in oil towards or away from a viewing surface in individual pixel cells to produce high contrast paper-like black or white image features with consistent appearance under wide viewing conditions and angles. Versions have included half black and half white particles, solely white particles in absorbing liquid, or both black and white particles [12]. Usually they are suspended in oil and encased in microcells between ITO electrodes, with a backplane. The devices can have threshold voltages, allowing implementation in matrix displays. E-Ink or ‘e-paper’ displays first came out of the Massachussetts Institute of Technology (MIT) Media Lab commercially in 1997 with the first microencapsulated electrophoretic film. Research efforts are working on multicolour displays [12]. A flexible electrophoretic display developed in 2017 produced plastic sheet displays with microencapsulated electrophoretic bubbles, and a 10% stretchable version using spacers to prevent collapse of the cells to maintain optical properties was demonstrated but not further developed [35], [36]. Solely structural stretchable approaches have been tried due to the liquid requirement- a soft structure would be subject to higher probability of leakage or short circuit under deformation. Another electrophoretic approach involves modulating whether or not light is totally internally reflected or transmitted from the display surface, from Whitehead’s group at the University of British Columbia [37]. It allows thinner (~10 µm) device structure and faster switching speed because the absorbing material only must move ~500 nm rather than the conventional 50-100 µm. The total internal reflection mechanism is achieved by microstructuring the surface to provide the necessary incident light angle. The different geometry and mechanism could potentially make designing the device stretchability easier, though the microstructure would still have to be adequately preserved under strain. 1.1.1.9 Electrochromic displays Electrochromic materials change colour between their oxidized and reduced states and include some metal oxides and organic materials such as polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) (PEDOT), and viologens, with a wide variety of performances and 10 colours [38]. In this section we give an overview of structure, properties, and the commercial potential of flexible electrochromic displays. Most common devices comprise two layers of transparent conductor ITO sandwiching an electrochromic material layer, electrolyte, and ion storage layer. Applying a voltage across the cell causes the electrochromic layer to reduce or oxidize and change colour, with a counter reaction occurring at the ion storage layer electrode and compensating ions traveling to each electrode through the electrolyte to supply necessary charge. The structure and layer requirements are discussed more in detail in 1.1.3 and in Chapter 4. In 2018, electrochromic glass that can be darkened with a flip of a switch was commercially available from companies SageGlass, Gentex and Switch Materials (a Simon Fraser University spinoff) in automobile rear view mirrors and sun roofs, and also implemented by Boeing and Ferrari [39], [40]. The high cycling stability of some of these rigid, highly encapsulated electrochromic devices has allowed fast integration into the market. Interest in commercializing electrochromics for rigid complex information displays has not been large due to the market saturation by liquid crystal technology, offering faster stable switching and facile production [41]. Earlier implementation in multi-segmented displays has been limited to research due to stability issues with liquid devices including corrosion, delamination, and electrolyte leakage. Currently, many cells use a polymer electrolyte that greatly improves device stability. Still, many electrochromics are slow to respond to voltage changes and lack a threshold voltage, causing crosstalk and challenging implementation in an array-based addressing system needed for multi-pixel displays, with details covered in more detail in Chapter 5. Where electrochromics offer a key advantage is in thin flexible information displays. Many flexible electrochromic displays have been developed on paper and plastic and exhibit low power consumption, switching speed down to milliseconds, high contrast, wide viewing angle, and good coloration memory, but they are still largely in the research stage. A few examples are shown in Figure 2a-b [10], [11], [40]. As it is a reflective device that only uses power while switching instead of the constant current required for luminescent devices, evaluation of performance is based on contrast change instead of luminescence. While there is no universal standard for comparison, coloration efficiency, or charge required to achieve certain contrast, contrast ratio, contrast change, or transmission or reflectance change are often used [12]. In 11 terms of mechanical performance, a recent PEDOT flexible device showed only 7.5% optical degradation after 1200 bending cycles [42]. Polymers offer the best advantage for development of multi-coloured devices because they can be structurally engineered to produce precise colour tunability and improved contrast [43]. PEDOT and derivatives also offer high stability, flexibility and contrast [44]. J. R. Reynolds, a leader in this area, has demonstrated multilayered active matrix devices used using pi-conjugated organic polymers with colours spanning the rainbow [45], shown in Figure 3 [46], [47]. Because of high contrast, stability, low power and colour availability compared to luminescent devices, electrochromics offer unique advantages for the development of stretchable displays. One of the first stretchable electrochromic devices was reported in 2013 with 50% strain operation and consisted of transparent silver nanowire-based electrodes with microscale WO3 Figure 3: Examples of thiophene-derived electrochromic films [46] reproduced with permission Figure 2: Flexible and stretchable electrochromic devices: a) flexible active matrix PEDOT display [10], b) [11] flexible PEDOT electrochromic display, c) stretchable WO3 electrochromic device [48] , d) electrochromic spandex in electrolyte [50] , e) intrinsically stretchable PEDOT/PU [5] reproduced with permission 12 electrochromic depositions on a polydimethylsiloxane (PDMS) substrate[48]. A stretchable electrochromic skin based on poly(3-hexylthiophene-2,5-diyl) (P3HT) was demonstrated in 2015 by deposition onto carbon nanotube transparent electrodes [49]. Electrochromic material deposited onto a fibrous conducting network integrated with spandex produced electrochromic 20% stretchable fabric [50]. All these previous versions involve depositing electrochromic material onto a stretchable fibrous conductive electrode, where the electrochromic layer itself is not stretchable and forms microscale deposits on the fibrous conductors, which sustain the stretching. This provides limitations in total strain and resolution depending on the stretchable network. In 2017 the first intrinsically stretchable electrochromic film was reported by Kai et al. based on a PEDOT-polyurethane (PU) composite film that served as both the electrode and the electrochromic material, demonstrating switching under 50% strain [5]. However, there was long switching time due to low conductivity and a fully integrated stretchable device with electrolyte and contacts was not shown. Examples of these stretchable devices are shown in Figure 2c-e. With recent developments of highly stretchable transparent PEDOT and highly conductive composites such as by Wang et al. in 2017 [6], we believe this could be a key to an excellent display system for lightweight wearable, skin-like devices that are intrinsically stretchable. Some of these advances will be covered more in detail in Chapter 2. Table 1: Most advanced towards intrinsically stretchable displays demonstrated with each technology Technology Structural only Intrinsic Max strain (%) Many colours High speed Low Power High brightness/ contrast High stability Ref. Phosphor X 700 X X [14] LED X 48 X X X X [17] LCD – – – – – – – – – OLED X 200 X X X [19] Quantum dot X 20 X X X [26] LEC X 120 X [31] Perovskite X 50 X X [34] Electrophoretic X 10 X X X X [36] Electrochromic X 50 X X X X X [5] The aspects of each stretchable display technology progress towards intrinsic stretchability mentioned are summarized in Table 1. Structural stretchability dependent on buckling of material 13 films deposited on prestrained substrates or fibrous conductive electrodes, or rigid islands with stretchable interconnects, will always exhibit limitations in maximum strain and resolution. Intrinsically stretchable active material-based devices instead will depend on how much the material itself can stretch while retaining performance. High brightness for electroluminescent devices is quantified as above 100 cd/m2 required for daytime viewing [12] while contrast is considered separately for non-luminescent displays. Given the above discussed advantages of electrochromics and intrinsically stretchable devices, we chose to focus on adapting electrochromics as a promising candidate for stretchable displays. In the next section we will provide an overview of the properties of electrochromic polymer PEDOT we chose to use. 1.1.2 PEDOT In this section backgrounds of PEDOT and common dispersion PEDOT:poly(styrenesulfonate) (PEDOT:PSS) are given to understand their basic structural, electrochemical, conductive, and optical properties. This will provide context for usage of PEDOT as our final device’s electrochromic, ion storage and electrode layers. In Chapter 2 more focus on PEDOT:PSS composite properties to tune electrical and mechanical performance is provided. PEDOT is our electrochromic polymer of choice given its high stability, contrast, speed, and derivatives providing potential for multi-coloured displays in the future. It also becomes our electrode of choice due to high transparency and conductivity. 1.1.2.1 Molecular structure We briefly discuss the molecular structure of PEDOT in order to introduce its electrochemical behaviour. PEDOT’s chemical structure is shown in Figure 4a, with the conjugated alternating single/double bond backbone shown in Figure 4b characteristic of conjugated polymers [51]. For each carbon in the chain, there are 4 valence electrons. Due to favourable energetic conformation, the first 3 settle into sp2 hybridized orbitals in plane with the backbone and form single σ bonds between the carbon atoms shown in red in Figure 4c. There is one remaining unhybridized out of plane pz orbital per atom shown in blue, and these participate in π double bonds. The π orbitals overlap and form a delocalized π orbital electron density cloud that propagates all the way down the conjugated backbone, as shown in Figure 4d. These π bonds 14 consist of both lower energy bonding π and antibonding π* orbitals formed due to interactions of the energy levels of adjacent atoms, where the remaining single π electron per atom occupies the lower π energy state, the other remaining empty. The high stability of PEDOT arises from the crown ether O-O bond that prevents side reactions at unwanted positions on the monomer [51]. 1.1.2.2 Electrochemical behaviour: molecular In this section we provide an overview of the electrochemical behaviour of PEDOT in terms of molecular theory. Its change in properties when oxidized or reduced gives rise to its electronic conductivity and optical switching behaviour. In the PEDOT:PSS section we go into practical study of electrochemical behaviour using cyclic voltammetry. PEDOT can be oxidized from its neutral benzoid form shown in Figure 4b to its oxidized quinoid form shown in Figure 4e through doping using chemical or electrochemical methods. With chemical oxidative doping, a chemical oxidant is added and removes an electron from the π orbital in the backbone, while in electrochemical doping, a voltage is applied to the PEDOT to remove electrons, leaving behind an excess positive charge. Counteranions A- come in and stay in close proximity to provide charge neutrality as in Figure 4e [52]. The polymer backbone can only incorporate further charges if they are compensated by an equal number of oppositely charged counter ions, such that dopants help enable redox charge transfers. In reduction the opposite occurs and the chain gains a negative charge and ions are expelled or cations come in to Figure 4: a) PEDOT structure, b) conjugated backbone, c) red sp2 hybridized orbitals in sigma bonds and blue π orbitals out of plane, d) delocalization of π orbitals along backbone, e) effect of oxidative doping with reference to [51] 15 compensate. Typically the PEDOT is synthesized in oxidized form with the counterions present as many polymerization methods are oxidative, as discussed at the end of this section. Conducting polymers are usually most easily p-doped as it is easy to remove the single π electron characteristic of the conjugated backbone [52]. The doping or dedoping is accompanied by changes to the electron cloud structure in the delocalized π orbitals along the chain, causing both electronic and optical effects. 1.1.2.3 Electronic properties This section discusses how the structure and oxidation state of PEDOT give rise to its electronic conductivity. We use PEDOT as our highly conductive electrode so it is important to understand how its conductivity is controlled and how it is affected by oxidation state. For each carbon atom, the filled π bonding orbitals form a delocalized electron density cloud along the conjugated backbone due to close proximity. In the neutral state, all the π levels are filled, but with oxidative doping and removal of many π electrons, an excess positive charge is created and becomes mobile, able to move up and down the chain due to the delocalized π electron orbital cloud. This is analagous to hole conduction in semiconductors. In solid crystalline materials, the outermost electron orbitals of closely packed atoms distribute to form valence and conduction bands that enable charge flow. The number of electrons filling each band and their separation determine whether a material behaves as an insulator, semiconductor or conductor. In our polymer, the π and π* energy levels of each atom overlap between adjacent atoms, and form energy bands analogous to a valence and conduction band [53]. Main differences include that the excess charge changes the positions of energy levels by distorting the flexible polymer chain, which enables conduction, and polymer lower level of order than semiconductor crystals creating a range of energy levels causing less defined band edges. The introduced mobile charges enabling conductivity and inducing distortions are called polarons or bipolarons. As shown in Figure 5 for a polaron, a single electron charge is removed or added from a energy level that moves into the energy gap up from the valence band or down from the conduction band. In most conjugated polymers, light doping creates polaron levels. A bipolaron is created at higher doping levels, where two polarons of the same charge become coupled, forming two levels in the energy gap. With even heavier doping, bipolaron bands form due to even more distortion of the conjugated backbone enabling increased charge flow due to 16 creating easily accessible energy levels. Eventually as doping increases, the energy gap disappears completely and doped conjugated polymers behave as metals with a continuous partially filled energy band where charges can freely flow [52]. Hence, in neutral undoped form these polymers are semiconducting, and highest conductivity is achieved at the highest oxidation. This means in our device, with hopes of using PEDOT as an electrode, it is important to test whether or not the PEDOT remains conductive enough in its reduced state to still act as an electrode or if it becomes insulating enough to prevent the cell from switching. The orientation of the PEDOT chains has a strong effect on electronic properties because it determines how the π delocalized electron density clouds overlap. PEDOT chains tend to form crystalline packing structures, such that the delocalized energy levels exhibit π-π stacking, and this provides high conductivity along the direction of the chains as the charges can easily move along the parallel backbones and jump between the chains [53]. Similar doping-dependent conductivity behaviour extends to all other intrinsically conducting polymers, including polyaniline, polypyrrole, and polyacetylene, which inherently range from semiconducting to highly conductive [51]. PEDOT as a transparent conductor has already been incorporated into several other devices including light emissive displays and photovoltaics [51]. 1.1.2.4 Optical properties Here we discuss the nature of PEDOT electrochromism, which arises from optical property change as the electronic band structure changes with oxidation level. A material that absorbs mainly in the red region (600-700 nm, 1.8-2 eV) and reflects or transmits in the blue/violet (400-450 nm, 2.8-3 eV) region will appear blue, whereas a material that absorbs in the blue region and reflects or transmits in the red region will appear more red. Figure 5: Change of electronic band structure in conducting polymers as they are further oxidatively doped until behaving like metals, adapted from [52] 17 For PEDOT, the electrochromic changes can be observed visually in changing between dark blue in the reduced to transparent in the oxidized state as the energy level separations are around the visible range. UV-Vis spectroscopy can be used to observe changes in spectral absorption changes. The spectrum of a typical PEDOT film is shown in Figure 6 , where a polymer film is immersed in electrolyte containing most commonly lithium ions, and is reduced or oxidized in an electrochemical cell configuration [51], [54]. In its oxidized (+0.5 V) state, it mostly transmits, appearing transparent blue, but as it reduces towards a peak at 2.2 eV (560 nm) grows and more absorption happens in the red region, making the film appear dark blue. This peak matching the bandgap indicates a transition in the PEDOT backbone associated with low doping shown in Figure 5b, attributed to polaronic states [52]. As oxidation level increases, this peak disappears and infrared peaks around 1.4 eV (886 nm) and 0.5 eV (2470 nm) appear from bipolaron states of heavily doped and conductive PEDOT, as in Figure 5c-d, making the polymer appear transparent as most visible wavelengths pass through [51]. For each electrochromic polymer the absorption spectra depend on the π-π* bandgap structure and can be tuned by interchanging different thiophene monomers or doping to create different colours. PEDOT films with best electrochromic performance have shown up to 45% transmission change for films 300-500 nm thick [55], [56]. 1.1.2.5 PEDOT:PSS Here, we talk specifically about PEDOT:PSS, a formulation that forms a stable dispersion in water, allowing for easier processability than PEDOT alone. This is the most used dispersion for Figure 6: PEDOT:PSS optical absorption spectra for different applied voltages in electrochemical cell, permission from [51] original from [54] reproduced with permission. Visible light is in the range of 1.8-3.1 eV. 18 electronics and is the one that we end up using for our device. We give a brief overview of the structure and conductivity mechanisms specific to this formulation, which become important in Chapter 2 to improve conductivity and stretchability. Conducting polymers exhibit similar behaviour to conventional polymer materials with the polymer chain configuration exhibiting some random walk behaviour, but the stability of the charge delocalization and conduction along the backbone makes it difficult to rotate or bend, such that the chains are rigid. All native state linear conducting polymers are unable to be shaped at low temperatures, unable to be joined with high temperature treatments, and form crystal phase structures. In order to increase processability, one must make modifications to support main chain flexibility through solubilizing a precursor or side chain substitution [52]. The most common combination is PEDOT combined with poly(styrenesulfonate) (PEDOT:PSS), allowing it to be applied from a water dispersion to a wide variety of technologies. PSS- is strongly acidic and polar and forms the excess host polyelectrolyte and counteranion for charge neutralization [52]. The anions are negatively charged and interact with the solvent, while the PEDOT chains tightly attach to them and do not interact with the solvent. This forms a necklace-chain structure in water with a core-shell structure: the PEDOT as the hydrophobic core and PSS as hydrophilic shell that enables easy dispersion in many aqueous and organic solvents like Figure 7a with believed chemical structure shown in Figure 7b [52], [57]. A stable PEDOT:PSS dispersion in water takes the form of microgel particles with a core of hydrophobic PEDOT surrounded by hydrophilic PSS [58]. PEDOT:PSS is available in commercial solutions from companies including Heraeus and Agfa, with a molecular weight ratio of PEDOT 1000-2500 g/mol and PSS 400 000 g/mol [51]. Figure 7: a) PEDOT (blue):PSS (yellow) coil structure in water dispersion, b) proposed molecular structure adapted from [52] 19 The dispersion can be processed using methods such as coating and printing. Because the PSS is highly acidic, it has PSSH segments for sections that don’t interact with PEDOT, and they dissociate into PSS and protons in water, where the PSS anions repel one another enforcing a linear conformation of the polymer. PEDOT:PSS without modification is amorphous, unlike PEDOT alone. PSS is insulating and so can greatly decrease conductivity of the PEDOT. In film form it has been observed that a stacked pancake structure is formed due to the shrinkage of the necklace ‘beads’ in the vertical direction, with PSS highly concentrated between the stacked layers [51]. The conductivity of the native PEDOT:PSS is dependent on the ratio of the two components, with more PEDOT supporting higher conductivity films. The separation of PEDOT sections makes the native PEDOT:PSS material of low conductivity ~1 S/cm [51]. The mechanism of charge transport between PEDOT sections and molecules is still not well understood. Dopants are added during processing or after to improve conductivity and mechanical properties through better connection between PEDOT regions, which is discussed more in Chapter 2. 1.1.2.6 Synthesis PEDOT can be synthesized using electrochemical, chemical, or vapour phase polymerization methods. In all cases the polymerization begins with the monomer 3,4-ethylenedioxythiophene (EDOT) that is commercially synthesized [51]. EDOT is very easily oxidized and protonated in the δ position of the thiophene ring by a strong acid as shown in Figure 8, forming an [EDOT-H]+ intermediate species that can then add to the basic carbon-2 on another EDOT. The same process can occur on the other side of the thiophene ring, allowing the formation of a dimer and then trimer, initially delocalizing the positive charge along the chain, such that oxidation can easily be used to convert EDOT to a chain with a stable conductive state. In a similar manner, oxidative polymerization in the presence of doping counterions is used to join the units to form Figure 8: Acid catalyzed dimer and trimer formation beginning PEDOT chain adapted from [51] 20 the polymer chain. This ease of being oxidized to a stable conductive chain makes this polymer uncommonly stable in air and water. Electrochemical polymerization occurs through cation radical polymerization, similar to Figure 8. Initially the EDOT molecules are oxidized by removing electrons by applying voltage to an electrode, such that they join into a chain as described. The solvent, surfactant and counter anions present affect the PEDOT morphology. PEDOT can be electrochemically deposited on metals [52]. Pei et al reported the first electrochemical polymerization and a native conductivity of 200 S/cm of PEDOT [44]. Chemical synthesis proceeds by adding EDOT and an oxidizing agent such as iron tosylate into solution. Polymerization and doping occur simultaneously, as the counter ions remain after performing oxidation to serve as dopants, meaning the PEDOT is immediately conductive up to reported 750 S/cm with iron tosylate [51]. Conductivity, reaction rate and resulting solubility are affected by reactant ratios. Vapour phase polymerization is like chemical polymerization but involves first coating or soaking the oxidizing agent onto the substrate. EDOT vapour is then released in a controlled manner into a chamber containing the substrate, such that oxidative polymerization occurs as EDOT molecules approach the surface from the air [52]. This can be used to control deposition of PEDOT onto solid or porous surfaces and can produce conductivities up to 1500 S/cm. 1.1.1.1 Electrochemical study We have introduced electronic and electrochemical properties at the molecular level, and here we introduce cyclic voltammetry and how it is used to study electrochemical behaviour of PEDOT and PEDOT:PSS on the macroscale. This is one method is used to evaluate switching voltage and charge capacity of our cell. Cyclic voltammetry is a common electrochemical method used to study reduction and oxidation of a material in an electrochemical cell [59]. In an electrochemical cell setup, the potential of a working electrode is linearly swept in a triangular wave over a specified range either across the whole cell or with respect to a standard reference redox couple like Ag/AgCl, and the current recorded. See Figure 9a-b to observe the shape of a general triangular wave applied and what current results for a redox couple potential scan (blue) and capacitive device (red) [59]. For a faradaic reduction or oxidation process, in an increasing potential, an anodic 21 current peak will occur at the oxidation potential as shown at point 2, with the reaction producing a current that is limited by the speed of electron kinetics between point 1-2 and by diffusion of species to the electrode between points 2-3, with similar behaviour in reverse for reduction. A reversible reaction will have the peaks separated ~60 mV at room temperature [59]. For a capacitive material that stores charge with increasing voltage, often in an electric double layer close to its surface, the current will be observed to exhibit no peaks but a broad plateau-like current, positive for increasing potential, and negative for decreasing potential. The positions and heights of the features in cyclic voltammetry depend highly on the speed and range of the sweep, reaction reversibility, and sample thickness and resistance. This method is useful for studying potentials at which complex electrode reactions occur under different conditions and the charge storage mechanisms occurring [59]. For PEDOT, in the forward scan, positive cations leave the PEDOT layer and are replaced by bipolarons injected from the electrode at an onset oxidation potential corresponding to the conduction band energy with the opposite happening in the reverse scan. As in Figure 9c there is broad redox signal that plateaus at high potentials, and exhibits some hysteresis, not being fully reversible [60], signal and peak positions can vary greatly depending on the properties and scan settings, in this situation showing difference for two different electrolytes [61]. This would be due to the differences in energy for ion insertion at these differing electrode-electrolyte interfaces. There are many theories on this behaviour. There is general agreement that there is likely a broad distribution of oxidation and reduction potentials throughout the material depending on individual chain energy levels and conformation [60]. The energy levels of each chain can be Figure 9: Cyclic voltammetry a) example of triangular potential scan, b) resulting voltammogram for reversible redox couple (blue) and capacitive structure (red), c) cyclic voltammograms of PEDOT:PSS films in different electrolytes, reproduced with permission from [61] 22 affected by polymer oxidation state because of changes in counterion insertion, swelling and solvation due to changes in hydrophilicity, or conductivity change with reduction, each of which can alter the polaron band structure. The potentials and currents at which oxidative and reductive charging occur vary widely with electrolyte and PEDOT preparation as there are different energies associated with different dopant and inserted ion conformations [54], [61]–[66]. The high capacitive current superimposed on the faradaic redox peaks is attributed to the high internal surface vs. volume ratio for electrically conducting polymers creating a large charge storage capability [60]. The mechanisms and ratios between the faradaic and capacitive charge storage processes are where researchers have not reached consensus, as they are difficult to separate. Among many others, Heinze et al. proposed different redox sites [60], some directly on the polymer electrode, and some involved in intermolecular bonds between adjacent chains so polarons and bipolarons can jump between them. Otero et al. and Aoki et al. have proposed relaxation processes that occur when charge is added or taken away due to stable conformations requiring overpotentials to break up, causing the hysteresis [60]. For the solely capacitive regime, Volkov et al. recently modeled the high capacitance as arising from an electric double layer at the interface between PEDOT and PSS [65]. Coupled ion electron diffusion and migration cause capacitive charging between 0.5 to -0.5 V. The capacitance in this case is measured to be 34 F/cm3, but this model no longer applies after a entering the faradaic regime. Additionally for PEDOT it is found that overoxidation occurs – for example above 1.4 V vs. SHE in H2SO4 [60]. It has a narrow bandgap and higher oxidation potential compared to polypyrrole and polyaniline with the ability to be both p and n doped [52]. In summary, PEDOT:PSS electrochemical behaviour has been studied with multiple mechanisms theorized for charge storage, both electrochemical and capacitive, some arising from interactions between the PEDOT and PSS portions. The behaviour is highly dependent on the structure, as this affects the energies and voltages required to add or remove charge. 1.1.3 Electrochromic cell structure Here we briefly go into the requirements for the electrochemical device layers to set the stage for our chapters exploring each step of integration of stretchability for each layer. General structures of electrochromic devices can be absorptive/transmissive, absorptive/reflective, or single layer [41]. For transmissive devices, light passes through or is 23 reflected off the device into the eye and if it is switched more light is absorbed, making contrast apparent. There may be two complementary electrochromic layers, with one colouring while the other goes transparent or changes to a different colour, or symmetrical, with a low transmissive state where one side is reduced and a high transmissive state where both exhibit intermediate transmittance, or one of the sides may be electrochromic while the other maintains a colourless state. Absorptive/reflective devices take advantage of different incident light absorption with switching causing different colour to be reflected and perceived. They are usually paired with an opaque electrolyte so only the top electrochromic layer is visible. For single layer devices, the electrochromic material is inserted into the ionically conductive layer where the redox reactions. As we base our structure on a PEDOT stretchable film, we are interested in a layered device that can be either transmissive or reflective. A typical structure of a layered electrochromic polymer cell using a polymer electrolyte is shown in Figure 10 [51]. There are top and bottom encapsulating layers, typically glass or plastic, and then sandwiching transparent electrode layers. The electrodes sandwich an electrochromic layer, polymer electrolyte and ion storage layer. The ion storage layer must be able to perform redox processes to counterbalance those happening in the electrochromic layer while contrast between two redox states can be observed and the electrolyte is necessary to transport charge-compensating ions. To perform the electrochromic switching, the electrochromic layer must be oxidized or reduced to change its colour. This happens in the cell as shown in Equation ( 1 ) and ( 2 ): Electrochromic layer: 𝑂1+𝐴𝑛−(𝑡𝑟𝑎𝑛𝑠𝑝𝑎𝑟𝑒𝑛𝑡) + 𝑒− + 𝐶𝑎+ ⇌ 𝑅1(𝑑𝑦𝑒𝑑) + 𝐶𝑎+𝐴𝑛− ( 1 ) Ion storage layer: 𝑅2 + 𝐶𝑎+𝐴𝑛−(𝑐𝑜𝑙𝑜𝑢𝑟𝑙𝑒𝑠𝑠) ⇌ 𝑂2+𝐴𝑛−(𝑐𝑜𝑙𝑜𝑢𝑟𝑙𝑒𝑠𝑠) + 𝑒− + 𝐶𝑎+ ( 2 ) [51]. The highly oxidized transparent electrochromic layer O1+ forms a complex with an anion An- to maintain charge neutrality. A negative charge is injected, changing it to its reduced Figure 10: Typical electrochromic cell structure 24 coloured R1 form, while a cation Ca+ comes in from solution to complex with the anion to maintain charge neutrality. But to provide the negative charge added when the colour changing material is reduced, at the other side of the electrochemical cell a species must be oxidized- this occurs at the ion storage layer. An initially reduced species R2 has an electron removed and is oxidized to O2+, which may be colour changing as well or colourless. To provide optimum performance, there are several considerations for each of the layers, discussed further in Chapter 4 [40]. Contrast, switch speed and stability are of utmost importance. The electrodes must be of high electronic conductivity to enable rapid transfer between the external circuit and electrochromic redox sites. The electrolyte having high ionic conductivity and low electrode-electrolyte resistance are very important to provide high speed switching, and the counterions must be of small enough size to intercalate into the active layers on each side rapidly. The electrochromic material must have high contrast, electrochemical stability and fast electron transfer kinetics with the electrode for good optical performance. For all layers, resistance to thermal and UV, corrosive and erosive effects is needed. In moving from flexible to stretchable electrochromic devices, there are many challenges to overcome. One is that each layer must be stretchable. Typically, the electrodes are made of indium tin oxide (ITO) on glass or plastic. ITO is transparent and conducting but highly brittle [51]. Recently, transparent nanomaterial composite electrodes, and PEDOT alone have been explored [40] as alternatives to ITO. PEDOT films are used as electrochromic layers and can often also be used as the ion storage layers, but in their native form are not highly stretchable. In the past few years stretchable PEDOT composites been reported. This advance has opened the possibility of using PEDOT to transport electronic charge, for colour changing and for ion storage. Four of the layers in Figure 10 are replaced by two PEDOT layers – one as the top electrode/electrochromic layer, and the other as the bottom ion storage layer/counter electrode, which can also be involved in the colour changing. There are several stretchable polymer electrolytes reported but they have not been more than minimally investigated for compatibility in stretchable electrochromic polymer cells. In order to form a stretchable electrochromic cell, all these layers must be made stretchable and adhere to one another to maintain the electrochemical interface under deformation, in addition to having durable contacts. An encapsulation layer is also needed – the substrate in Figure 10 – which must also be stretchable. Finding, characterizing 25 and assembling the set of stretchable layers needed for an electrochromic device and then characterizing performance is the main challenge of this thesis. 1.2 Overview In this chapter we have introduced the project and motivated the electrochromic approach to stretchable display technology. We have also given some background on fundamental properties of conducting and electrochromic polymers. PEDOT is an example, which we use for our exploration, as described in the coming chapters. We have also described the components needed to be designed for our stretchable electrochromic device – namely stretchable electrochromic, ion storage, electrode, electrolyte and encapsulation layers, some which we end up combining. The challenge is to find compatible layers that can be brought together to form a fully functional and stretchable electrochromic device. In relevant previous work, stretchable electrochromic devices based on depositing non stretchable electrochromic material onto stretchable structures have been demonstrated, as well as one example of an intrinsically stretchable film but not a full device. The structural approach is limited by the supporting material’s deformation limit and can have negative impacts on the appearance and resolution due to buckling or other geometrical features. Our goal is to instead create an intrinsically stretchable display device with integrated contacts by attempting to make all the layers in the device intrinsically stretchable, electrochemically and mechanically compatible, and able to operate under strain. This is highly challenging because any of these materials will potentially experience changes in electrical, chemical and mechanical properties under strain and it is difficult to predict how this will affect the operation. The design requirements are as follows: • Substrate: a transparent elastomer is needed. It must encapsulate and support the active materials inside the cell. • Electrodes: they must be stretchable but remain electrochemically stable in the operation voltage window, while enabling efficient conduction and high transparency. • Electrochromic layer: it must be stretchable and have high contrast, high electrochemical stability and fast electron transfer kinetics to ensure good optical performance. 26 • Electrolyte: it must have high stability, high ionic conductivity, low electronic conductivity, high stretchability and form a strain-durable high-quality electrochemical interface with the electrochromic layer. • Ion storage layer: it must be stretchable and may use the same electrochromic material. • Contacts: the interconnects to external hardware must be chemically stable and mechanically durable. To address each of these requirements, we approach the components sequentially and combine them to build an intrinsically stretchable display and multi-pixel array. In Chapter 2 we replicate and test stretchable PEDOT composite films on transparent elastomer substrates for electrochromic and stretchability properties and then choose the best candidate. We also test dual usage as transparent electrodes and the ion storage layer and confirm suitability. Chapter 3 covers the synthesis of stretchable, high ionic conductivity electrolytes and the choice of one that forms electrochemical and mechanical interface compatibility with the PEDOT layer. Chapter 4 involves bringing the previous results together in the design, fabrication and characterization of a standalone stretchable device in single pixel form and development of contacts that are chemically and mechanically stable. In Chapter 5 we present a 3×3 passive matrix display and demonstrate its addressable, low cross talk operation under 30% strain. Finally, we conclude with by describing the intrinsically stretchable electrochromic device, describing its advances over other research in the field, and discussing potential applications along with the next steps that could be taken to advance the research. Overall, our product accomplishes the original goal, with demonstration of an intrinsically stretchable electrochromic device in single pixel and multi-pixel format. It demonstrates that all the layers are chemically, mechanically and electrically compatible with one another and the device operates under 30% strain with minimal performance degradation. Performance approaches other similar flexible devices in the field with the added benefit of stretchability. 27 Chapter 2. Stretchable electrochromic film In this chapter we explore fabricating stretchable PEDOT composites with both elastomers and doping agents. These composites enable combined conductivity and stretchability. We investigate these composites as intrinsically stretchable electrochromic layers and whether or not they can also function as a transparent electrodes – replacing brittle ITO. We explore two previous approaches to impart intrinsic stretchability to PEDOT: 1) adding ionic dopant plasticizers lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM OSU) shown to provide both high conductivity and stretchability in PEDOT:PSS films [6], 2) combining PEDOT with polyurethane elastomer to provide conductivity and stretchability. We fabricate PEDOT:PSS/ionic dopant composite films, confirm their enhanced properties, and additionally show they are electrochromic and conductive enough to be used additionally as electrodes to switch their redox states. We fabricate the elastomer composite and confirm reports of its stretchability, conductivity and electrochromism. We select the BMIM OSU ionic liquid/PEDOT:PSS composite as the best candidate for integration into devices, due to its high conductivity in addition to acceptable optical and mechanical properties. These properties enable usage as the combined electrode, electrochromic, and ion storage layers, allowing us to make a simplified device structure without need for additional stretchable transparent electrodes, as a step towards building our full stretchable electrochromic device. 2.1 Background In the background for this chapter we cover details of previously demonstrated PEDOT composites that impart enhanced conductivity and stretchability through the addition of dopants or cosolvents and mechanisms by which enhancement occurs, and a composite that imparts stretchability through addition of elastomer polyurethane. 2.1.1 Stretchable and highly conductive doped PEDOT:PSS 2.1.1.1 Doping In Chapter 1 we touched on doping of conducting polymers and that it can be used to increase the conductivity and mechanical durability of PEDOT:PSS. One of the approaches to stretchable 28 PEDOT:PSS explored is in adding dopants that act to both increase conductivity and stretchability; this eventually allows us to use the polymer as both the electrochromic and electrode layer. This combination of functions into one layer is important for stretchability because indium tin oxide (ITO) is the conventional transparent conductor layer used for electrochromic devices and it is not stretchable. To understand how these properties are enhanced, it is first important to understand how the doping of PEDOT:PSS enhances conductivity. As mentioned in Chapter 1, native PEDOT:PSS forms microgels in water with a PEDOT-rich core and PSS-rich shell. In a film, they form a flattened pancake-like stacked structure with conductive PEDOT regions isolated from one another by insulating PSS regions , such that that native film conductivity is as low as ~1 S/cm [67]. Adding ionic or cosolvent (secondary solvent to water) dopants to PEDOT:PSS by mixing or post-film surface treatments changes the way the PEDOT and PSS interact. Most enhance conductivity in a similar way, with the molecules or ions interacting with one or both of PSS- and PEDOT+ electrostatically, screening their coulombic attraction and introducing more phase segregation and nanomorphological changes [6]. This causes a PEDOT conformational change from benzoid, associated with tightly coiled macroscopic structure, to more positively doped quinoid structure, which is associated with more linear and loosely coiled structures as seen in Figure 11. It induces a more interconnected PEDOT network with increased π-π stacking and nanocrystal formation, enhancing charge delocalization, and improving conduction through the network [52], [68], [69]. Treatment also removes some insulating PSS chains, reducing the ratio of PSS to PEDOT causing conductivity increase and increasing material porosity [52]. Some examples of dopants used to improve PEDOT:PSS conductivity from native ~1 S/cm include organic cosolvents, organic solids, ionic liquids and surfactants, with some listed in Table 2 [51], [52], [70]. Cosolvents such as ethylene glycol or DMSO screen PEDOT and PSS electrostatic interactions due to high dielectric constant rather than ionic interaction [68]. For Figure 11: Alternation between benzoid and more linear quinoid structure, adapted from [52] 29 ionic compounds containing cations that easily donate electrons, the cations bind to PSS-, and in some cases the anion can also bind with positive charge on PEDOT chains, screening PEDOT and PSS coulombic attraction [52]. Sometimes the PSS- can be completely replaced by different smaller anions, increasing conductivity. Many acids are found to enhance conductivity, thought to be due to protonation of PSS- reducing its PEDOT coulombic attraction and changing hydrophilicity, further contributing to phase segregation [51], [52]. Recently, ionic dopants have enabled production of PEDOT:PSS with conductivity above 4000 S/cm [6], [71], breaking records for conducting polymers aside from polyacetylene. Table 2: Highest PEDOT:PSS conductivity adding dopants as of 2015, adapted from [52] Additive σ (S/cm) Dimethylsulfoxide (DMSO) 1000 Ethylene glycol (EG) 1460 Ionic liquids 2084 Alcohols 1000 Acids >3000 2.1.1.2 Doping and plasticization Native PEDOT:PSS films are flexible, but strain as low as 6% is able to cause fracture [72]. This number can be greatly increased with plasticizing additives. Recent reports of simultaneously highly conductive and stretchable freestanding films are the enabling factor for our intrinsically stretchable device. Here we give an overview of PEDOT plasticization mechanisms and dopants that cause both increased conductivity and plasticization. In polymers, plasticizers increase free volume in the films and weaken intermolecular forces between polymer chains, allowing greater compliance [73]. In polyelectrolyte complexes such as PEDOT:PSS they induce swelling of the material, weaken electrostatic attraction between the components due to screening, enhancing the free movement of polymer chains [72]. Plasticized PEDOT:PSS exhibits mostly plastic rather than elastic deformation, causing formation of microbuckles upon relaxation. High compliance samples tend to show a nanofibrillar surface structure, associated with PEDOT and PSS phase segregation, but not necessarily correlated with conductivity increase [73]. Adding plasticizing but insulating surfactants such as Zonyl or Triton X can increase stretchability to up to 25-40% [73]–[75], but do not necessarily increase 30 conductivity. It has been found that mechanical properties also depend strongly on pH and humidity, as these additionally affect degree of hygroscopic PEDOT:PSS swelling [51]. Of main interest for our project, some ionic dopants that act to increase conductivity also greatly enhance the stretchability of PEDOT:PSS. We drew from an impressive result reported by Wang et al. of up to 176% stretchability for free standing samples of PEDOT:PSS ionic dopant composites accompanied by a drastic reduction in Young’s modulus, unparalleled by previous results [6]. The films were observed to have fibrous conductive networks similar to the other plasticized samples – as is evident in Figure 12b-d when compared to non-stretchable PEDOT:PSS/DMSO of Figure 12a, where in AFM phase measurements, contrast can show difference between crystalline and amorphous regions. Unlike the surfactant plasticizers, they also enhanced connections between PEDOT regions, with achieved conductivities >4000 S/cm, potentially also due to PSS removal through anion replacement with the dopant anions. Figure 12: Phase images of PEDOT:PSS with dopants a) DMSO b, c, d) conductivity and stretchability enhancers, e) stress strain behaviour, and for STEC1 f) stress/strain behaviour, g) relaxation time, h) storage and loss modulus, i, j) theorized nanomorphology change, reproduced with permission from [6] 31 Figure 12e-h show the mechanical properties of the typical films, found to be initially elastic in stress vs. strain up to around 20%, followed by largely plastic deformation. The shallow slopes of the doped samples indicate a drastic decrease in Young’s modulus to levels able to be stretched by hand. The relaxation time with 2% strain is shown to be on the order of 72 s, indicating the plasticization occurs slowly as the polymer chains rearrange. The stress-strain cyclic curves show a degree of irreversible deformation, as the stress in the reverse mechanical cycle is less for a given strain than the forward strain. The ratio of storage to loss modulus is low, indicating viscoelastic behaviour. Thus we expect that though the material is observed to stretch above 100%, a lot of it is plastic deformation and not elastic behaviour, and must happen on a long time scale. This does not prevent us from designing a device but means that there will be a degree of permanent deformation upon strain and it is not purely elastic. The mechanism by which both conductivity and stretchability are improved is still poorly understood, theorized to be due to a combination of oxidative doping and plasticizing actions of these dopants [6]. Generally, it was observed that anions with higher acidic or electron-withdrawing properties or higher mobility than PSS tended to better enhance conductivity, theorized to be due to their doping of the PEDOT and anion replacement of PSS, verified to occur on the surface [6]. Dopants were mixed separately with NaPSS and PEDOT and found to produce a gel in both cases, indicating that these dopant plasticizers had plasticizing effects on both PEDOT and PSS. It was theorized that samples with the highest conductivity had the highest crystallinity, forming a network of PEDOT regions due to phase segregation by the dopant plasticizers, as can be seen from the white fibers in the AFM phase images associated with more crystalline materials. Instead of increasing the rigidity, as is typically expected of higher crystallinity, the plasticizers are distributed in the amorphous regions causing swelling and charge screening, increasing chain motion range and increasing compliance, as pictured in Figure 12i-j [6]. It should be emphasized that these are proposed mechanisms only. Though many of these films have been reported for high conductivity and stretchability, we did not find any ionically doped films studied for electrochromic behaviour. We appear to be the first to use this highly stretchable and conductive composite for electrochromic purposes, especially in using it as both an electrode and electrochromic material. For the ionically doped film we chose to test the highest conductivity and stretchability dopant, lithium 32 bis(trifluoromethanesulfonyl)imide (LiTFSI), as well as the highest stretchability dopant also producing high conductivity, ionic liquid BMIM OSU from [6]. 2.1.1.3 Ionic liquids as dopant plasticizers From the aforementioned results, the highest strain was reported for the addition of ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (BMIM OSU) [6]. As we were looking for an electrochromic film that could produce the highest strain, it was of interest to learn if there were some specific enhancements ionic liquids could provide and below is provided some additional literature review on this topic for PEDOT:PSS doping. Ionic liquids are low temperature molten salts that have unique aspects including high chemical and electrochemical stability, low flammability, low vapour pressure, and tunable ionic strength, and because of these properties are often used as electrolytes in electrochemical devices [67]. Several studies have been done on their interactions in composites with PEDOT:PSS [67], [72], [76]–[78], and generally it is found that the counterion exchange occurs to different extents depending on the ions and concentrations of the ionic liquids. In situations with high counterion exchange, similar phase segregation and nanofiber, crystal formations are reported to other ionic dopants. The large organic imidazolium cations typical of ionic liquids participate in cationic exchange, and often render the polymer hydrophobic, causing precipitation in water due to a high degree of interaction [78]. Strongly electronegative hydroxyl groups on imidazolium rings may also hydrogen bond with the sulfur cation S+ on the thiophene ring [6]. With a unique advantage of tunability and stability of ionic liquids, the ions can be exchanged to tune for the desired particular strength of ionic interaction [77]. In particular, 1-ethyl-3-methylimidazolium tetracyanoborate (EMIM TCB) has been studied as a plasticizing dopant for PEDOT:PSS by several groups [67], [77], producing conductivity >2000 S/cm and stretchability 50% [72]. Similar to the other ionic dopants, large EMIM cations associate with the PSS backbone replacing H+, neutralizing it and allowing more flexibility of the chain, swelling PSS regions increasing free volume and inducing better connection of the PEDOT regions through phase segregation [67]. There is evidence of anion replacement of PSS by smaller TCB, causing closer crystalline packing of PEDOT chains [77]. Grazing-incidence wide and small angle x-ray scattering revealed crystal lamellar spacing in PEDOT:PSS of ~20 Å in the (100) plane, and interplanar PEDOT ring stacking distances of ~3.5 33 Å in the (010) plane, compared to nanofiber diameters 25-55 nm, meaning there were multiple stacks across the fiber, with smaller fibers found for smaller anions [77]. Due to ionic interactions it was found to precipitate with PEDOT immediately after mixing in specific concentrations >2% weight [67], [72]. Overall, it is apparent that ionic liquids can act similar to other ionic dopants and some may increase plasticization. We chose to test a film using BMIM OSU, reported for high strain in PEDOT:PSS composites [6]. The structure is shown in Figure 13a. We might expect the BMIM cation to participate in cationic exchange similar to EMIM, causing the PSS to take up large organic cations and increase free volume and phase segregation between PEDOT and PSS. The OSU is not as small as TCB so we might expect the degree of PSS replacement to be less, but the long chain could potentially increase some hydrophobic properties, disrupting nanofibrillar formation due to its large size. 2.1.1.4 Processing and testing considerations As reported by several studies, conductivity and stretchability enhancement can be highly dependent on the processing, dopant concentrations and film treatment conditions, making it difficult to reproduce results reported by other groups. It is theorized that in deposition from dispersion, the doped PEDOT:PSS does not immediately reach equilibrium: thermodynamics over time drive arrangement into an energetically favourable nanostructure [51]. Various studies report that dopants can change the physical properties of the PEDOT:PSS dispersion to a gel-like state due to electrostatic interactions [6], [51], [72], [79]. As ionic strength increases, there are two distinct phase transitions, as depicted in Figure 13b, where whether or not it presents as Figure 13: a) Molecular structure of BMIM OSU, b) dependence of PEDOT:PSS phase on dispersion ionic strength with dopant additive. From [79] with permission 34 liquid, gel and liquid, or gel depends on ionic strength and concentration. It is found that additionally that the PEDOT:PSS microgels first decrease in size and then increase again. As ion concentration increases, the PSS chains initially stiffen due to coulomb repulsion, experience more ion insertion and become more flexible due to charge screening and lower aggregation. Later the particles get larger again due to repulsion between individual microgel particles, eventually reaching a high enough concentration where the repulsion breaks down, the particles aggregate, and some polyelectrolyte is precipitated, leading to physical gelation. This of course changes the PEDOT:PSS molecular interactions and will have a large effect on the phase segregation, morphology, and, consequently, the conductivity, when it is deposited as a film. However, there was no direct connection between the phase and physical gelation observed with conductivity enhancement, and the mechanisms are still poorly understood, but implications for film fabrication can be identified. With this very precise phase transition behaviour, and the fact that the highest conductivity enhancements tend to be achieved at ionic concentrations just below gelation [67], [72] it is clear that small concentration variations can have large effects on the solution in preparation, as once the dispersion aggregates, it becomes very difficult to achieve thin uniform films via spin coating and spray coating. As many references state, the strain at which film cracking appears for PEDOT:PSS films supported by an elastomer substrate depends on preparation and testing conditions, including humidity, surface treatment, strain rate, temperature treatment, and film thickness [73], [80]. According to fracture mechanics, imperfections can cause localized cracking, while the majority of the film remains pristine under strain [6]. Very small step sizes are often used to allow for long time plastic deformation and relaxation of 1% steps or 10% per minute [6], [72], [81], [82] and we follow this method as well. 2.1.2 Stretchable PEDOT and elastomer composites We sought a stretchable electrochromic film, so we also looked at combining PEDOT with an insulating stretchable elastomer material, such that the composite retains conductivity and electrochromism from the PEDOT and elasticity from the elastomer. Most elastomers are hydrophobic, whereas PEDOT:PSS and EDOT are more hydrophilic, such that any combination with hydrophobic elastomer without modification leads to isolated PEDOT islands that cannot 35 connect to conduct [83]. Two ways to get around this are to use either more polar elastomers or modify hydrophobic elastomers to be more hydrophilic. The only other reported intrinsically stretchable PEDOT/elastomer electrochromic film mixes EDOT with thermoplastic polyurethane (PU) and then chemically polymerizes PEDOT, using tosylate as a doping anion [5]. The composite is highly stretchable >200%, moderately conductive (200 S/cm) and able to electrochromically switch under 50% strain [5], [84]. It was observed to phase separate into PEDOT and elastomer regions at PEDOT contents of 20% or 40%, with 30% content being stably homogeneous, indicating some threshold concentration for stable suspension [84]. Other examples of PEDOT elastomer composites with PEO, PVA and/or PDMS reported strains up to 75%, phase separation at certain mixture thresholds, but lower conductivities [83], [85], [86]. We chose to test the PEDOT/PU composite and achieved high contrast and stretchability, but due to its low conductivity we opted to continue developing the high conductivity ionically doped version in our final device. 2.2 Fabrication 2.2.1 Materials All chemicals were used as received. For the ionic dopant samples, PEDOT:PSS PH1000 dispersion 1-1.3 wt% in water from Heraeus was used. BMIM OSU, LiTFSI, tetrahydrofuran (THF), EDOT, toluene, LiClO4, propylene carbonate (PC) and butanol were analytical grade purchased from Sigma Aldrich. Capstone fluorosurfactant was purchased from Dupont. SEBS elastomer Tuftec H1052 was obtained as a sample from AK Elastomer. For the PEDOT/PU composites, thermoplastic polyurethane Chronothane T 80A shore hardness from AdvanSource Biomaterials was used. Iron tosylate 40 wt% in butanol was purchased from Heraeus. PDMS Sylgard 184 was obtained from Dow and Corning. Glass slides were cut in three 2.54 cm squares and sonicated for 1 h each in acetone, ethanol, deionized water and then stored in propanol. ITO glass 70-100Ω/□ was purchased from Sigma Aldrich. 2.2.2 PEDOT:PSS DMSO composites PEDOT:PSS/DMSO composite films as described in [74] were used as a non stretchable control for comparison of PEDOT film conductivity and electrochromism as this is a common 36 formulation used to make conductive (900 S/cm) transparent spin-coated films. To make these films, the PH1000 dispersion was mixed at 700 RPM for 15 minutes with 5 wt% DMSO and 1 wt% Capstone fluorosurfactant. 400 μL was spin-coated at 400-1000 RPM onto square glass slides, with the added fluorosurfactant enabling wetting and uniform film formation, and then the samples were annealed on a hot plate at 130º C for 15 min. 2.2.3 PEDOT:PSS ionic dopant composites The PEDOT:PSS ionic dopant highly conductive and stretchable films were fabricated as in reference [6] with some changes applied. We tried the highest conductivity and stretchability dopant, LiTFSI, and the highest stretchability while moderately conductive dopant, BMIM OSU. For stretchable samples, to form the substrate as in the previous report, 20 wt% SEBS was dissolved in toluene for 48 h. The solution was cast onto glass slides and dried for 48 h in ambient conditions and then at 70º C for 4 h. 1.0 g of PH1000 was diluted by 100 mg deionized water. Typically, 100 μL of 10 wt% dopant in deionized water solution was added while stirring at 1600 RPM for 10 min. These conditions were obtained after iterative tests to improve film uniformity, thickness and conductivity, while avoiding gelation discussed in 2.1.1.4. The cast SEBS film, glass or ITO glass substrates were treated with oxygen plasma at 30 W for 3.5 min (Harrick Basic Plasma Cleaner PDC-32G), or UV-Ozone for 30 minutes to perform hydrophilicization, enabling uniform films. 400 μL of the dispersion was spin coated on the surface of the substrate at 1000 RPM for 60 s, followed by annealing on a hot plate at 130º C for 10 min. 1 mL of 1 wt% ionic dopant in deionized water was dropped onto the film surface, allowed to sit for 1 min, and then the film was spun at 3000 RPM for 60 s and annealed on a hot Figure 14: As prepared films: a) 46% BMIM OSU/PEDOT:PSS on glass, b) 20%, 30%, 40% PEDOT/PU on glass, c) 46% BMIM OSU/PEDOT:PSS on SEBS, d) 30% PEDOT/PU on PU 37 plate at 70º C for 20 min to remove dopant and PSS to increase conductivity. Examples of films are shown in Figure 14a, c. 2.2.4 PEDOT elastomer composites The 30% PEDOT elastomer composites were fabricated as in [5] with some changes applied. Polyurethane was dissolved at 8 wt% in THF overnight. EDOT, 40% iron tosylate in butanol, butanol, and 8% PU in THF at 0º C were combined in a 0.126:0.929:0.286:5 weight ratio and stirred at 1000 RPM for 5 min. 400 μL of solution was spun on glass or ITO glass at 1000 RPM for 60 seconds and then polymerized at 100º C on a hot plate for 5 min, where the film turned from yellow to dark blue. Unreacted monomer and oxidizing agent were rinsed away with deionized water and then the film annealed at 70º C for 20 min. The ratio of added EDOT was varied to produce 20-40% PEDOT films as well. Films are shown in Figure 14b. To prepare samples that were used to test strain, glass slide substrates were coated in a thin layer of hydrophobic PDMS monomer prior to film fabrication. A PU film and PEDOT/PU film were treated with oxygen plasma at 30 W for 3.5 min, then clamped together at 70º C for 2 h to induce plasma-assisted thermal bonding [87], [88]. The substrate could then be peeled off with the PEDOT/PU film attached. A prepared film is shown in Figure 14d. Through our collaborators at the University of Cergy-Pontoise we used Magenta electrochromic polymer reported previously in [47] to try to make another stretchable electrochromic composite with polyurethane to provide multi-colour capability, with details provided in Appendix A. 2.3 Characterization 2.3.1 Morphology We used an Asylum MFP-3D AFM in tapping air mode to investigate nanomorphology of our films using AC240TS Asylum tips. 2.3.2 Electrical For quick relative comparison of film conductivity, two-point probe measurements of resistance were taken to compare R/□ between samples, but this could not be used for film conductivity due to inclusion of contact resistance. 38 Four-point measurements were used to find conductivity values of selected films. Measurements were taken using a custom designed spring-loaded gold coated 4-pin setup with pins spaced by 2.54 mm that was clamped lightly onto the 2.5 cm square samples so that all pins were in contact centered on the film. A current of between 1-5 mA was run from one outer pin to the other using a Solartron 1287 potentiostat galvanostat or an Autolab PG101 potentiostat, and the voltage measured between the middle two pins. The Asylum AFM was used to measure the thickness of the films by creating a step at the edge and used in the conductivity calculation, or a Dektak profilometer. 2.3.3 Mechanical For strain tests, PEDOT:PSS films <1 μm thick are too delicate to handle alone with strain localization rapidly causing fracture with any film imperfections or strain nonuniformity [81]. Depositing thin films onto a stretchable substrate spreads the stress, substantially slowing strain localization, and this is a method employed in several thin film stretchability tests, with crack onset strain observed through an optical microscope [6], [80]. We used a home built 3D printed clamp and screw mechanism designed to fit under the optical microscope as pictured in Figure 15 such that we could observe crack formation during stretching. This was subjective, requiring judging when similar amounts of cracks had formed and not just ones due to defects. The quantity of tests (5-9 per similarly prepared sample type) helped suggest which films tended to perform best on average. Substrate surface treatment and film thickness along with inconsistent manual controlled strain rate are expected affect the failure strain. Figure 15: Mechanical strain setup for crack onset measurements 39 2.3.4 Electrochemical Electrochemical testing of films was carried out using a Solartron 1287 potentiostat galvanostat in 1 M LiClO4/PC in a 3 electrode configuration to test switching behaviour of films by applying voltage and recording currents. Cyclic voltammetry scans were run using Ag/AgCl as a standard reference electrode and carbon paper with surface area ~4 cm2 as counter electrode. Tests were run at speeds of 20, 50 and 100 mV/s. Step responses were measured with 1 V steps to compare time constants and charge transfer characteristics. PEDOT films for these tests were deposited on ITO glass. Alligator clips were used to contact the top corner of the films without electrolyte contact. 2.3.5 Optical A Cary UV-Vis 5000 spectrophotometer was used to evaluate transmission for film oxidation and reduction. The film was immersed in 1 M LiClO4/PC and attached to a 3 V battery with counter electrode carbon paper for 20 seconds to reduce or oxidize the film, followed by drying and immediate measurement using glass as a baseline, repeated 2-3 times. 2.4 Results 2.4.1 Morphology Under optical microscope the films of PEDOT:PSS with ionic dopant were similarly homogeneous. The feature variation was on a smaller scale, better studied via AFM. In Figure 16 we observe the height and phase images for samples doped in 3 different ways. As described in [89], areas where the phase is closest to 90º are the most dissipative, indicating material softness and less crystallinity. As evidenced by the phase range, we are in the repulsive or high amplitude <90º phase range, so lighter regions are higher dissipation and likely softer PSS regions while darker regions are likely more PEDOT rich. Between the phase images a and b the dark regions appear possibly better connected in a network structure. In the BMIM OSU there are larger regions of each type of phase, with different domains also apparent in the height images, indicating potentially higher phase separation, consistent with previous results [6], [67], [90]. Compared to previous studies showing increase of fibrous structure, we do not see evidence of this. This could be limited by the noise resolution, tip sharpness and residual dopant or water 40 causing swelling- especially in sample c with the ionic liquid that is nonvolatile and could be sitting on the surface. In Figure 17 we observe the AFM images of PEDOT/PU for different ratios of PEDOT and PU. The height images show some differences, but it is the phase images that provide the most information. In the higher and lower concentrations of PEDOT there is phase separation between Figure 17: AFM height and phase images different ratios of PEDOT to PU in PEDOT/PU composites- height is in m, phase in º Figure 16: AFM images of PEDOT:PSS composite films with a) 5% DMSO, b) 46% LiTFSI, c) 46% BMIM OSU, phase images scaled to same º range, 1 μm image width 41 two regimes- PEDOT and PU, leading to high contrast in the phase images, consistent with previous results for a similar composite but in different ratios [84]. Due to the theorized strong molecular interactions between these materials due to chemical structure, it is shown here as it has been previously that in an optimal ratio there is no phase separation between the two separate components- the variation in phase for the middle ratio is minimal. The ratios may be different due to the different polyurethane used. The large phase range indicates jumping between the attractive and repulsive regimes despite the smooth topography of the films, indicating dissimilar mechanical regions in the film. 2.4.2 Electrical We report minimum two-point resistance measurements for films 2.54 cm×2.54 cm to compare relative R/□ in Table 3. Based on these measurements, and Equation ( 3 ) we estimated the conductivities of the DMSO, LiTFSI and BMIM OSU films to be >200 S/cm with a typical film thickness of 200 nm and the PEDOT/PU films to be >1 S/cm with a typical film thickness of 1.5 μm, from several AFM measurements, but not taking into account contact resistance which could have a large effect due to typical rectifying contact formed between metals and PEDOT [91]. This is consistent within an order of magnitude to conductivity reported in previous results for PEDOT/ionic dopants as reported in [6] and 5-10 S/cm reported for the PEDOT/PU film [5]. The LiTFSI and BMIM OSU sample both showed highly conductive results and could reduce and oxidize on glass without the ITO in solution, showing they could potentially be conductive enough in their reduced state to act as electrodes. The PEDOT/PU sample was highly resistive compared to the doped PEDOT:PSS and could not reoxidize on glass alone, so would not be suitable as a dual electrode/electrochromic layer, so its conductivity was not further investigated. Table 3: Two-point film resistance measurements and approximate fracture strain Film Minimum R/□ Highest strain with ~30% film surface cracking 5% DMSO/PEDOT:PSS 800 Ω N/A 46% LiTFSI/PEDOT:PSS 1300 Ω 16±6% 46-67% BMIM OSU/PEDOT:PSS 280 Ω 27±5% 30% PEDOT/PU 50 kΩ >80% 42 As we found consistently better results with BMIM OSU/PEDOT:PSS in our mechanical tests as described below, we later chose to only measure four-point conductivity of this sample compared to the control DMSO composite. We used Equation 1𝜎=𝜋𝑙𝑛(2)𝑡 (𝑉𝐼) 𝑓2 ( 3 ) to correct for geometrical probe spacing, we calculated the R/□, where V and I are voltage and current, σ is conductivity, thickness t<<s the probe spacing and f2 is a geometrical correction factor corresponding to w/s=2.5/0.3 and l/w=2 or 1 depending on film geometry where w is width and l is length, obtained from a lookup table [92]. Figure 18 demonstrates an example of a profile taken of the edge of a BMIM OSU/PEDOT:PSS film on glass using AFM for one of the measurements made used to obtain film thickness. Results for best films are shown in Table 4 and are close to previous reported best values of 900 S/cm [74] for the DMSO film, and 420 S/cm [6] for the BMIM OSU film. Table 4: Four-point film conductivity measurements Film Thickness Conductivity 5% DMSO/PEDOT:PSS 150 nm 735 S/cm 46% BMIM OSU/PEDOT:PSS 137 nm 447 S/cm Conductivity under strain was difficult to measure due to the difficulty of making reliable contacts to the PEDOT layer that held up under strain. Fracture first happened at high strain Figure 18: Example film thickness measurement – right: AFM height image 80 µm wide with a profile taken along the red line, left: height profile showing ~600 nm step of PEDOT to glass, x axis in µm across the sample 43 regions close to the contacts making resistance increase measurements unreliable. In [6] researchers used liquid metal based contacts to make careful contact under strain but we did not design an apparatus for this. In Chapter 4 we were able to measure increase in resistance of the electrode with strain in the final cell after support for the contacts and support for the film in form of PVA electrolyte were developed. 2.4.3 Mechanical Measuring strain of cracking onset is complex as it requires making a judgment of when ‘real’ crack onset has appeared not just due to defect induced localized differences in strain. We were only able to achieve an approximate judgment of onset and so observed our films under optical microscope and approximated when cracking was apparent over approximately 30% of the film- usually at this point it was over the entire film and not just localized strain regions. An example of film cracking used to judge fracture can be seen in the 25% strain image of two films in Figure 19. We estimate an uncertainty of 5% strain for all strain measurements for crack onset due to qualitative nature of the judgment and potential uncertainty due to sample slipping in the clamps while under strain. The images are only examples and are not the best samples. We imaged a PEDOT/PU typical film at 0% strain, then strained to 33% and observed no cracking. At this point we increased magnification and strain to 82%, observing still no cracking at which point we reached the maximum range of our strain stage. We do know that the PEDOT/PU exhibits cracking after strains >100% by observing a previous strained film. While attempting to optimize parameters for high stretchability, we collected datasets with this information for many samples. We then compared results from several similarly prepared samples and took an average of the strain and standard deviation to judge what were the best performers. Given the qualitative nature of the judgment of true crack onset and not just cracks from localized imperfections, it is interpreted as an approximate judgment of which films consistently performed most promisingly to justify continuing to the next stage alongside other factors including electrochromic, electronic performance. Our values are summarized in Table 3 with uncertainty being the standard deviation. The values were averaged over 9 measurements of 46-50% LiTFSI films similarly prepared and 5 similarly prepared BMIM OSU films- both 47% and 67% BMIM OSU/PEDOT:PSS ratios were included as no significant difference in 44 mechanical durability was observed. Evidently, PEDOT/PU has drastically higher stretchability without visual cracking evident. Despite the qualitative nature of these measurements, BMIM OSU showed several more promising strain experiments than LiTFSI. However, no trials came close to the >100% crack onset strain for similarly prepared films from [6]. Many parameters were varied in attempt to get closer to this value but overall, none were found to have a profound enough effect to greatly improve the stretchability to the level reported in the reference. It was found that diluting the dopant prior to mixing had a great effect on improving film uniformity, preventing gelation, and possibly improving stretchability by ~5-15%. Plasma treatment time was found to have minimal effect, except at treatment over 4 minutes slightly worse result was observed. Ratio of dopant to PEDOT:PSS up to 77%, heat pre-treatment time and temperature of substrate, hot plate post-treatment temperature, vortex vs. stir plate agitation, degassing, were all found to not have a clear effect on the film stretchability. Dopant concentration above 77% produced immediate gelation preventing uniform film formation. Two-point resistance values were also found to vary widely Figure 19: Example films under strain through optical microscope, left: doped films (irregularities due to impurities or spin coating nonuniformity), showing initial cracks in film ~20% strain, and cracking over entire film at 25% strain, right: PEDOT/PU film on PU strained initially on 5X magnification, then increased to 20X, showing no clear evidence of mechanical degradation up to 82% strain, features include air bubbles between PEDOT/PU layer and PU from bonding 45 without evident correlation, later attributed to possible rectifying contact between the PEDOT and metal contacts, but potentially also due to the film being damaged during probe contact. There were evidently varying parameters with much larger influence on film properties than the variables we were controlling for. One reason proposed for the drastic stretchability variation could be the mechanical straining procedure. In [6] straining films was controlled at 10%/minute with 5 min rest intervals per 10%. Strain localization due to usage of the custom device built to fit under the optical microscope was not able to strain as uniformly or at as controlled a speed as a dynamic mechanical analyzer to accommodate for long (72 s) relaxation time. This could cause development of localized thinning or initiate cracking prematurely due to uneven stress. It is possible that if we strained using the prescribed conditions our strain would be larger, but we did not have the ability to image with the microscope under these conditions. It is also possible there were unidentified contaminants in films or substrate leading to localized strain developments or that the ratios of dopant were not correct, but this still does not account for the dramatic 70% different in fracture strain onset. Later strain experiment with the full devices showed no film cracking under 40% strain but this was also when in full mechanical contact with the electrolyte providing stress distribution and support. Overall, the discrepancy remains unexplained but did not prevent us from proceeding to create a multi-layered intrinsically stretchable device. 2.4.4 Electrochemical Confirming the electrochemical behaviour of the films during reduction and oxidation will help to confirm the voltages needed to rapidly create high contrast changes for our final device. Here we validate our film electrochemical behaviour on ITO before proceeding to device integration to confirm adequate performance in liquid electrolyte. In Figure 20a, b we observe the step response of our films to a +1 and -1 V voltage step with respect to counter electrode carbon paper. This is not predicted to be exactly symmetrical due to expected non-zero open circuit potential existing between PEDOT, ITO and carbon paper without initial application of constant zero voltage to discharge initially. We used this to compare the relative speed and charge responses between the films. We observed for all the films the behaviour is relatively similar for the first 9 cycles. By taking the area under the current curves in a, b we calculated the charge transferred, though this may not account for the total charge that 46 would be transferred if holding the potential for a longer period. By taking the time for the value to decay to 1/e of the peak value, we can get the electrical time constant. This is related to but different from the optical time constant, measured in the later chapters, as the optical change may take a different amount of time to propagate across the entire film surface as a result of resistance along the film in both charging and discharging processes, as the metal connection is only at the edge. Because the film is ~5× thicker, the PEDOT/PU film transfers the largest amount of charge, though its exact PEDOT mass is unknown. However it is also the slowest, likely due to the higher resistance material and thicker layer preventing fast charge and ionic conduction through the vertical layers, and lower potential initially felt at the electrode-electrolyte interface due to some voltage drop over the high resistance film, slowing down the reaction. Compared to the LiTFSI, the BMIM OSU shows slightly higher charge transfer and slower speed, but as the films are of different thickness and uniformity, this was attributed potentially to sample variation. The speed is of concern for fast electrochromics, so the faster changing ones are preferable: LiTFSI or BMIM OSU. The PEDOT/PU films could be faster by making them thinner as well but they were not conductive enough to warrant optimization in the desire to use the electrochromic layer also as the electrode. Figure 20: a), b) 1 V step response of films on ITO showing time constant and thickness; cyclic voltammograms of c) different PEDOT films at 100 mV/s, d) LiTFSI/PEDOT:PSS 47 In Figure 20c, d we observe the cyclic voltammograms and redox waves for our different composite films exhibiting different oxidative and reductive behaviour. As mentioned, the redox peaks’ prominence and position can vary widely with solution, dopants and preparation. The PEDOT/PU shows a sharp reduction peak around -0.7 V with respect to Ag/AgCl, consistent with some other studies, and a small oxidative peak between 0-0.3 V, dominated by a capacitive oxidation wave. This fits with previous results that conducting polymers are expected to have a range of redox site energies as well as capacitive storage, causing the peaks to be very broad and provide an unclear difference between capacitive and faradaic current processes. For the LiTFSI doped film the peaks are shifted slightly closer to one another, which could indicate slightly lower energy for lithium ion insertion and deintercalation or a smaller IR drop. For BMIM OSU the reduction appears to occur at a lower voltage, the peak not evident, and the behaviour appears much more capacitive, perhaps with a lower overall capability of charge storage as indicated by the smaller area of the curve, but this could be attributed to the reduced PEDOT content reducing the prominence of these peaks (33% vs. 47% PEDOT:PSS). As well, the consistency of film thickness at this point was uncertain so the area decrease could be due to the content of PEDOT difference- this was not precisely determined. At close to 1 V vs. Ag/AgCl we observe the expected upwards curve for mainly LiTFSI and PEDOT/PU films indicating the initial stages of PEDOT overoxidation that happens starting around this voltage value [93]. Interestingly this is less prominent in BMIM OSU/PEDOT:PSS films, perhaps indicating a shift in the second oxidation potential due to dopant intermolecular interactions. In Figure 20d is an example of a comparison of different scan speeds for an ionically doped composite. According to reference [59] for a capacitive response, current scales linearly with scan rate and in a kinetics limited faradaic response, and current also scales linearly with scan rate for a reactant confined to the electrode surface. This is a possible explanation for the approximate linear increase with scan rate of the current for both the charging and reaction current, though theoretical studies still have not fully separated the behaviours for PEDOT. For all films tested, the redox peaks did not appear to move with the increase in scan speed. This indicates that the electron transfer kinetics are fast, otherwise we would observe motion of the peaks with scan rate [94]. We don’t look in detail here at the charge transfer to compare with charge per mass of PEDOT reported in literature- we look at this in Chapter 3 when cycling in combination with polymer 48 electrolytes. Overall, electrochemically our films appear to perform as expected, reducing and oxidizing similar to other PEDOT electrochromic films indicating electrochemical suitability for integration into electrochromic devices. 2.4.5 Optical We measured optical performance both visually and quantitatively using a UV-Vis spectrophotometer to confirm suitability of film electrochromic performance. Figure 22 shows an example of a ionically doped PEDOT:PSS film and PEDOT/PU film in reduced and oxidized states on ITO in LiClO4/PC. It is noted that the wetting of the ionic dopant film on ITO was poor, such that as the film was drying it shrunk from the edges. Nevertheless, a rich blue colour is produced for a film thickness of approximately 200 nm. The PEDOT/PU film being approximately 1.5 μm thick typically showed an initial darker colour and very rich contrast, but this may be mainly due to increased PEDOT content- we cannot make a quantitative comparison as to coloration efficiency without films of similar initial thickness or absorption. Additional tests were done with the polymer films on glass without ITO to test if the PEDOT layers could be used both as electrochromic and electrode layers- success was found with the high conductivity ionic dopant films but the PEDOT/PU could not reversibly switch between the Figure 21: UV-Vis spectroscopy measurements of films with typical thickness t. left: ΔTmax= 47% (603 nm), middle: 36% (620 nm), right: 34% (555 nm) respectively Figure 22: Photos of electrochromic films in oxidized and reduced states, a) LiTFSI/PEDOT:PSS b) PEDOT/PU 49 two states without the support of ITO- there is more detail on this in Chapter 3 in testing with polymer electrolytes. The UV-Vis spectrophotometer measurements of transmission in oxidized and reduced states for different films are shown in Figure 21. The ionic dopant BMIM OSU and LiTFSI ratios to PEDOT:PSS are different from the BMIM OSU film as this was before ratio optimization, however the optical qualities of the films were very similar. There is likely some uncertainty due to residual electrolyte on the films but overall our film performances of ΔTmax of 34-47% appear well within the range of well performing PEDOT films (reported up to 45% for films 300-500 nm) [55], [56], adequate to justify implementing in a device. 2.5 Discussion In this chapter we identified that out of our tested choices, a BMIM OSU/PEDOT:PSS composite film was a potential candidate for an intrinsically stretchable electrochromic display device that could act as both an intrinsically stretchable electrode and the electrochromic layer, exhibiting stretchability before significant fracture of up to ~27% on SEBS transparent substrate and conductivity >400 S/cm, sufficient to reduce and oxidize in electrolyte without need for an additional transparent conductor layer. It also gave comparable electrochromic and electrochemical performance to other PEDOT films in literature presenting it as a candidate for a high performing PEDOT electrochromic device. In the next chapters pair this film with a highly stretchable and ionically conductive electrolyte, substrate and interconnects to form a full vertically encapsulated electrochromic device. 50 Chapter 3. Stretchable electrolyte In this chapter we test our electrochromic films using polymer electrolytes, consider the requirements for a stretchable electrolyte, and then fabricate and test several to choose one for our structure paired with our BMIM OSU/PEDOT:PSS electrochromic film from Chapter 2. We investigate high ionic conductivity and stretchable polymer electrolytes: 1) a polyethylene oxide/nitrile butadiene combined network swollen with LiClO4/PC or EMITFSI, 2) a polyvinyl alcohol/phosphoric acid solid composite. We fabricate them, confirm their conductivity and stretchability, then test them electrochemically in combination with our PEDOT electrode layer to see if they can produce high speed and contrast electrochromic switching. We also then investigate whether they can form a mechanical bond with the electrochromic layer necessary for maintaining the electrochemical interface under strain, critical for the next stage of development into full layered devices. 3.1 Background 3.1.1 Electrolytes in electrochromics In electrochromic devices the electrolyte choice is important as it allows the ions to flow through the electrochromic and ion storage layers to provide compensating charge for the redox reactions required to change the electrochromic film colour. The choice of electrolyte affects kinetics and degradation mechanisms in the electrochromic cell. In this background section we discuss options for stretchable polymer electrolytes for our electrochromic display. There are some basic electrochromic electrolyte requirements [40]. High ionic conductivity is important so that the electrochromic switch speed is not limited by ion migration or diffusion rate, and low electronic conductivity to avoid short circuits. The electrolyte needs to be confined to define the device active area and physically separate the electrochromic and counter ion storage sides of the cell, while providing cations and anions to counterbalance redox reactions at each electrode and maintain charge neutrality. Conventionally liquid electrolytes are used, usually a salt that dissociates into ions in a solvent, but this increases chance of leakage, short circuiting, nonuniform pressure distribution, and low electrochemical stability due to material migration. Lithium ions are common due to small size and high diffusion rate, electrochemical stability and high tendency for electrostatic 51 interactions due to high charge density improving charge transfer at interfaces, as well as extensive research in lithium ion battery architecture, whereas hydrogen cations – though fast – can reduce to hydrogen gas easily [95]. Other alkali ions are also sometimes used. Aqueous electrolyte potential windows are too small due to hydrolysis of water, so solvents like propylene carbonate are better as they provide a larger potential window for operation with low parasitic reactions. Ionic liquids offer advantages as they serve as both solvent and ion sources [40]. They have high ionic conductivity up to 10 mS/cm, high transference number, large electrochemical stability window, nonvolatility, favourable viscosity, and temperature stability. They have also shown enhancement of electrochromic device stability and cycle life. 3.1.2 Polymer electrolytes for electrochromics For a stretchable device, due to concerns of liquid electrolytes, the optimal practical design is solid state, so gel polymer and solid electrolytes are the optimal candidates and are currently being optimized for use in electrochromic devices [96]. In this section we describe common polymer electrolytes used in electrochromic devices. Polymer electrolytes are easy to process into thin films, can act as both separator and electrolyte, are durable, have a wide temperature range, and reduce reactivity due to their solid nature. They provide electrochromic devices with higher contrast and memory and are often improve layer adhesion and durability to electrochromic layer swelling and deswelling with ion intercalation [96]. Some additional requirements exist specific to polymer electrolytes for electrochromic devices. An ionic conductivity on the order of 1 mS/cm or better is needed to achieve performance on par with liquid electrolyte systems and the material must have the ability to donate electrons in order to interact with cations. It should have solvation capability, good optical qualities, and be durable [40], [96]. There are several different types of polymer electrolytes, but what they all have in common is a salt dispersed in a polymer matrix that screens the ions, and the ions move between ion coordination sites within the matrix [96]. Solid polymer electrolytes are solvent free, where the ionic conduction is formed by salts dissolved in a polar polymer matrix and conduction occurs with ion motion through high free volume amorphous regions [40]. They are usually prepared via solvent evaporation and many have low ionic conductivity of ~0.001 mS/cm. Polymer gel electrolytes additionally contain a liquid plasticizer that swells the polymer, creating a two-phase 52 system with liquid solvent through which ions can travel and solid polymer matrix. They tend to have higher ionic conductivity ~1 mS/cm or higher but poorer mechanical durability. They can be viscous or crosslinked. Composite polymer electrolytes are made by adding inorganic material such as Al2O3 or SiO2 or ionic liquid to improve mechanical and electrochemical properties. A few polymers are commonly used for electrochromic electrolytes. Polyethylene oxide (PEO) and polyethylene glycol (PEG) systems permit high ion mobility with ionic conductivity up to 0.1 mS/cm above 40º C, but have high crystallinity at room temperature, limited temperature range, and can create a poor electrode-electrolyte interface due to chemical properties [40]. Usually, researchers add salts such as LiClO4, polar plasticizers and sometimes microscopic fillers, which reduce crystallization, allowing higher ion mobility. PMMA has also been studied extensively with polar, amorphous and flexible structure enabling high ion conductivity (~10 mS/cm), durability, high transparency, stability, diverse solvents and salts, easy processability, high quality electrolyte-electrode interface, and good electrochromic cyclability [40]. PVDF-based electrolytes, possessing both crystalline and amorphous regions, have also been developed with high ~1 mS/cm conductivity with durability, easy processing in solvents, high hydrophobicity, stability, thermal stability, flexibility, easy salt dissociation, and wide potential window [40]. 3.1.3 Stretchable gel electrolytes For our stretchable device, we seek a stretchable polymer electrolyte that meets the above requirements, and a Young’s modulus of <5 MPa to enable stretching by hand. Many highly stretchable gel electrolytes have been reported recently but only a few might qualify. Hydrogels would be candidates if they were compatible material-wise with our film, as many are highly ionically conductive (~100 mS/cm) and highly stretchable (>3000%) [97], but because the PEDOT:PSS is water dispersible, aqueous electrolytes quickly cause degradation. Several stretchable and highly ionically conductive electrolytes have been reported. For a few examples, PEG with imidazolium-based ionic liquid reported ionic conductivity 8.5 mS/cm and strain up to 280% [98]. A network composite of PEO and nitrile butadiene rubber (NBR) swollen in EMITFSI from our collaborators demonstrated 400% strain and 1 mS/cm conductivity [99]. Notable PMMA gel electrolytes reported 0.6-0.78 mS/cm and 400-700% strain [100], [101] or 1-10 mS/cm and >100% strain [102]. A PVDF-HFP based electrolyte that was crosslinked and 53 swollen with EMITFSI reported 100% strain and conductivity of 1.4 mS/cm [103]. Other examples containing swelling solvent include an acrylate-based rubber that showed 19.7 mS/cm and 1600% strain [104], and a polyepichlorohydrin composite that showed 0.2 mS/cm, and 600% strain [105]. Evidently, plasticization can be used to impart significant stretchability to polymers while not preventing them from being highly ionically conductive and we have a few options to consider for trial. Because it was developed by our collaborators, we chose to try the PEO/NBR semi-interpenetrating network high ionic conductivity and strain electrolyte. 3.1.4 Polyvinyl alcohol Among highly ionically conductive and stretchable polymer electrolytes, composites based on semicrystalline polyvinyl alcohol (PVA) are discussed here in more detail as it is the electrolyte we use for our final device. A simply prepared PVA (Mw>85 000 g/mol)/H3PO4 composite in a 1:1.5 weight ratio reported 3.4 mS/cm ionic conductivity, 410% elongation, and 2 MPa Young’s Modulus, with only 5% plastic deformation after 1000 stretch cycles to 100%, relying on physical crosslinking and entanglement for elasticity [106]. The electrolyte can be considered as solid, as all the water solvent is evaporated. PVA has been briefly used before in electrochromic cells but that was discontinued due to electrochemical stability issues [107]. Pure PVA is rigid with crystallized layers, but adding acids or other specific chemicals plasticize it and render it highly ionically conductive due to intermolecular reactions screening charge between the PVA chains, for acids in the form of protons bonding to OH groups [108]. This increases the free volume and amorphous behaviour, increasing ionic conductivity and plasticity. Though it has favourable ionic conductivity and stretchability, the stability window is narrow, creating potential problems for our device stability. Protic electrolytes have a voltage operation window limited by hydrogen evolution as shown in 2𝐻+ + 2𝑒− ⇌ 𝐻2(𝑔) 𝐸0 = 0.00 V 𝑣𝑠. SHE ( 4 ) [109]. However, a higher concentration of salt increases the potential window because the local pH near electrodes changes, shifting the position of the hydrogen evolution reaction according to the Nernst equation: 54 𝐸 = 𝐸0 +0.059𝑉𝑧𝑙𝑜𝑔10𝑎𝑂𝑎𝑅 ( 5 ) where E0 is the standard reduction potential, E is the reduction potential, z is the charge, and aO and aR are the concentrations of the oxidized and reduced species. A lower pH for more acidic electrolytes could widen the potential window approximately 56 mV per pH unit. Increase in lifetime observed of some PVA electrolyte electrochemical devices when water hydrate or water is removed is observed [110]. Hence, we still decide to try this electrolyte. 3.2 Fabrication 3.2.1 Materials Propylene carbonate (PC), LiClO4, polyethylene glycol methyl ether acrylate (PEGM Mn=500 g/mol), PEG dimethacrylate (PEGDM Mn=750 g/mol), PVA (Mw=146k-186k), cyclohexanone, TiO2 anatase powder, and 85 wt% H3PO4 in water were obtained from Sigma Aldrich and used as received. EMITFSI was from Solvionic. Nitrile butadiene rubber (NBR) with 44 wt% acrylonitrile from Lanxess and dicyclohexylperoxydicarbonate (DCPD) from Sigma Aldrich were used. Silver epoxy paint was obtained from SPI. Materials used in film fabrication described in 2.2.1. 3.2.2 Semi-IPN The PEO/NBR semi-interpenetrating network (semi-IPN) was a stretchable, high ionic conductivity polymer gel electrolyte selected and fabricated according to [99] with some modification with assistance of our collaborators at the University of Cergy-Pontoise. NBR was dissolved in cyclohexanone for 12 hours in a 1:4 weight ratio. EMITFSI was added and stirred overnight in a 1:2.5 NBR:EMITFSI wt ratio. PEGM and PEGDM were mixed in a 3:1 wt ratio then the mixture added to the NBR in a 2:3 weight ratio and stirred for 30 minutes. DCPD was added in a 3 wt% vs. PEO ratio and stirred for 15 minutes. It was degassed for 20 minutes and spread between two glass plates spaced by 250 μm thick TeflonTM, and then placed in the oven at 50º C for 3 h and 80º C for 1 h for radical initiated polymerization. The resulting membrane was vacuum dried for 3 days. An alternate version was fabricated following the same steps but using AiBN as radical initiator, and the temperature treatment was altered to 70º C/1 h and 90º C/3 h for higher activation temperature [111]. 55 We also tried to replicate previous results bonding PEDOT:PSS with the semi-IPN following [112] to form a bilayer or trilayer. PH1000 was mixed in 40 wt% PEO mix vs. solid PEDOT:PSS ratio then 400 μL cast on a glass slide or spun at 1000 RPM (60 s) and annealed at 70º C for 1 h. The PEGDM in the PEO mix was expected as in the previous report to serve as a crosslinking agent, binding the PEO in the PEDOT:PSS to the PEO based electrolyte, forming a strong bond. The semi-IPN precursor solution was cast on top and then the same temperature treatments above were performed using a hot plate under argon, in attempt to cross-link the PEDOT:PSS with the semi-IPN, a process previously used to build trilayer actuators. We repeated this with similar 20:20:60 wt ratio of BMIM OSU:PEO:(PEDOT:PSS) to test bonding. 3.2.3 PVA/H3PO4 The PVA/H3PO4 stretchable electrolyte was prepared as in [106] with modification. 10 wt% of 146-186 k Mw PVA was dissolved in water at 95º C for 2 h, then 85% H3PO4 in water was added to form a 1.5:1 wt ratio between H3PO4 and PVA, which was then stirred for 12 h at room temperature. For TiO2 samples, 10 wt% TiO2 in water was added so that there was 15 wt% TiO2 vs. PVA. The solution was cast and evaporated in ambient conditions for 48 h, then under vacuum for another 24 h. To test bonding we cast on PEDOT:PSS/BMIM OSU films. The fabrication of the 3-layer device is covered in 4.2. Copper tape joined with silver paint epoxy from SPI was used to form contacts to each exposed side of the PEDOT:PSS/BMIM OSU films but was shielded from the electrolyte with epoxy. 3.3 Characterization 3.3.1 Mechanical For the semi-IPN, Young’s modulus and strain of this material had already been characterized (0.69 MPa, 400% max strain) by our collaborators [99], but we confirmed strain >100% by manually stretching for samples 220 µm×2 cm×1 cm. A BOSE Electroforce Dynamic Mechanical Analyzer with a 22 N load cell was used to measure the Young’s modulus of the PVA/H3PO4 by applying 2 and 5 mm (0 to peak) sinusoidal strain displacement, up to 3 N of force at 0.1 Hz for a sample ~3 cm×1 cm×200 µm. A linear actuator with a LN-Drive motor driver controlled by an Arduino UNO and custom code was used to apply strain up to 250%. Manual peel tests were performed with tweezers to confirm whether the electrolyte adhered well 56 to the PEDOT layer, with subjective judgement of whether any resistance was provided to peeling the layers apart with small applied forces. 3.3.2 Electrical To obtain ionic conductivity, a film of electrolyte was clamped using binder clips, with minimal electrolyte compression, between two stainless steel plate electrodes with measured area of contact 2 cm2 and ~200 µm thickness. Electrical impedance spectroscopy (EIS) using a Solartron 1287 potentiostat galvanostat and Solartron Analytical 1260A Frequency Response Analyzer was performed from at least 1 MHz to 5 Hz (10 mV signal around OCP) to obtain the impedance spectra, allowing derivation of the resistance across the material at high frequency as in [113] according to the equivalent circuit model shown in Figure 23. Here Rs is the series resistance of the electrolyte, while the capacitance is from the electric double layer and ion adsorption, Rct due to charge transfer resistance of any Faradaic current transfer, and W a Warburg element due to ion diffusion. At high frequency the capacitive components have negligible impedance and we only measure Rs. As high frequencies can also demonstrate inductive behavior, we confirm the region we use shows approximately constant impedance and zero phase for the region in which resistance is estimated (the x intercept). The ionic conductivity is calculated according to 𝜎 =𝑡𝐴𝑅𝑠 ( 6 ) where t is thickness, A is area of exposed electrodes and Rs is the series resistance measured from the x intercept of the Nyquist plot. 3.3.3 Electrochemical Cyclic voltammetry was performed with a Solartron 1287 interface in a three-electrode configuration at 10 mV/s for the PVA electrolyte, for which potential stability window had not been previously reported. Tests were first done with aqueous H3PO4 while increasing the Figure 23: Circuit model for electrical impedance spectroscopy measurement, adapted from [113] 57 concentration to observe the effect of the phosphoric acid concentration on potential window, using Ag/AgCl as a reference electrode and carbon black/SIS composite (covered in Chapter 4) or ITO as the working electrode. For solid electrolyte tests, the polymer solution was cast in contact with carbon black electrodes and water evaporated using methods described above, forming a sandwich cell as in Chapter 4 with potentials then applied in cyclic voltammetric 10 mV/s scans in a 2 electrode setup to verify the potential window. Electrochemical step tests were carried out by clamping PEDOT on the substrate with an electrolyte separator and ITO or another PEDOT counter electrode, with copper tape contacting the PEDOT film at a single edge of each of the working and reference electrodes. Electrolytes tested included semi-IPN using DCPD or AiBN soaked in EMITFSI and LiClO4/PC, as well as PVA/H3PO4. Voltage steps were applied using a Solartron 1287 potentiostat galvanostat or a VSP potentiostat from BiologicSA. Varying 2 electrode configuration potential steps of up to 3 V were applied for reduction and oxidation, with voltages varied to produce maximum apparent contrast and speed. Performance was judged according to contrast, speed, charge transferred, and maximum currents, with additional consideration of stability range. 3.3.4 Optical Visual comparison of electrolytes was done. When testing electrolytes with switching electrochromic films, optical contrast was approximately evaluated from the video according to L*a*b* visual colour and contrast standards, though this was not very quantitative for these early tests due to inconsistency in imaging conditions. This colour conversion is discussed in 4.3.1.1. 3.4 Results 3.4.1 Semi-IPN 3.4.1.1 Mechanical Both semi-IPN’s made with DCPD and AiBN were able to be strained by >100%. We assumed the previous results to be correct of the same polymer electrolyte reporting 0.7-0.8 MPa Young’s modulus for strain in the linear elastic region of <50% and 400% elongation at break, within appropriate range for implementation in our device [99]. None of the semi-IPN PEDOT:PSS bilayers with attempted crosslinking passed the peel test- there was no resistance to peeling the electrolyte from the PEDOT layer with no manually 58 detected adhesion. Contrary to the trilayer actuators reported in the previous work, strong bonding was not achieved. This could be due to an effect of the thinner PEDOT:PSS layers, or processing variations such as temperature, surface treatment, or chemical concentration. The surface of the AiBN polymerized semi-IPN in the bilayer was also heterogeneous, which may have interfered with bonding. Overall these results indicate our PEDOT:PSS layers did not crosslink to the electrolyte via the PEGDM in the PEDOT:PSS. In future, other ratios of precursors, crosslinker and treatment conditions could be explored to try and overcome this bonding challenge. 3.4.1.2 Electrical The intercepts of the Nyquist plots in Figure 24 are used to calculate the ionic conductivities of the DCPD membrane to be 0.27 mS/cm and the AiBN membrane to be 0.22 mS/cm, comparable to previous reported results of 1 mS/cm [99], with thicknesses ~220 μm and area ~5 cm2. 3.4.1.3 Optical Comparing the optical qualities of the electrolytes was important as they would be visible due to the transparency of PEDOT in its oxidized state, and overall device contrast is important. Qualitative comparison of the electrolytes seen in Figure 24b-c found that the DCPD semi-IPN was red-yellow, and so potentially undesirable due to its optical qualities, while the AiBN version was tinted whiter. The DCPD version was more transparent than the AiBN version, which might be more suitable for an opaque device where the contrast is produced from the top Figure 24: a) Nyquist plots AiBN and DCPD initiated electrolytes showing x intercept to calculate ionic conductivity, b) DCPD semi-IPN, c) AiBN semi-IPN 59 electrode only with a reflective white electrolyte as a background. In Figure 25a-c we observe the contrast obtained for switching a single electrode using the semi-IPN membrane as a separator, a and b showing the effect for a single BMIM OSU/PEDOT:PSS film, and c showing the higher contrast capability of PEDOT/PU. In Figure 25d-f we observe the contrast produced by backlit devices using BMIM OSU/PEDOT:PSS as counter electrodes, enabling all-polymer active layers. For the BMIM OSU films, though the AiBN electrolyte version appears lower contrast, the lighting conditions are not the same- the photos are taken on a backlit background and the AiBN is more opaque. Quantitative comparisons require standard and referenced lighting conditions, which are not addressed until Chapter 4 experiments, so from these images it was unclear which one performed better optically, but it is assumed the AiBN electrolyte would be better suited to reflective electrolyte opaque reflective devices given its lower transparency. The BMIM OSU samples have less visual contrast compared to the PEDOT/PU samples perhaps due to less PEDOT material or inability to reduce as much due to strong oxidative doping by the ionic liquid. Between the single and double PEDOT electrodes, there is a decrease in brightness and contrast due to masking by the additional blue layer, with the PEDOT/PU still exhibiting the highest contrast. From this it appears all these combinations could be viable candidates. Figure 25: Cells in reduced and oxidized states using semi-IPN electrolyte a) (BMIM OSU/PEDOT:PSS)/DCPD semi-IPN/ITO, b) (BMIM OSU/PEDOT:PSS)/AiBN semi-IPN/ITO, c) PEDOT/PU on (BMIM OSU/PEDOT:PSS)/DCPD semi-IPN/ITO, (Reduced: -3 V, Oxidized:+1 V across cell), d) (BMIM OSU/PEDOT:PSS)/DCPD semi-IPN/(BMIM OSU/PEDOT:PSS), e) (BMIM OSU/PEDOT:PSS)/AiBN semi-IPN/(BMIM OSU/PEDOT:PSS), f) PEDOT/PU on (BMIM OSU/PEDOT:PSS)/DCPD AiBN/(BMIM OSU/PEDOT:PSS) (Reduced: -3 V, Oxidized: 0 V across cell) 60 3.4.1.4 Electrochemical For the EMITFSI-based semi-IPN electrolyte, the stability for the liquid solvating electrolytes was previously reported to be 5 V (-2.4 to 1.7 vs. SHE), larger than carbonates, which report -2.3 to 2.2 V vs. SHE stability in electrolytes containing common lithium salts [114]. A typical data set is shown in Figure 26 for a single electrode test with LiClO4/PC swollen semi-IPN between BMIM OSU/PEDOT:PSS on glass and ITO, first applying -3 V and then +1 V for reduction and oxidation, showing peak currents followed by exponential decay as unreacted material is used up. The Nyquist plot shows a small double layer capacitance and charge transfer resistance semicircle, with cell resistance of 787 Ω, indicating low barrier for charge transfer, such that it may not limit electrochromic reaction speed. The L* plot is mainly qualitative given the inconsistent video conditions and camera movement, but can be used to estimate within 3 s uncertainty the optical time constant, defined where 90% of the contrast change occurs [40]. In Table 5 we compare the electrochemical step performance of films with different semi-IPN electrolytes with an ITO counter electrode. As previously reported, bare ITO has extremely limited charge storage capability limited to its electrochemical double layer which cannot contribute adequate charge in the counter reaction to allow full switching of electrochromic material at the working electrode, such that any appreciable switching will contribute to reactions at the counter electrode that degrade the ITO [115]. Despite this, ITO is still commonly used without an ion storage layer to initially test electrochromic materials, but at the cost of ITO degradation. The films were not highly consistent in thickness or treatment, and contact quality with copper tape and electrolyte is expected to vary, such that without a larger number of test samples we can only make approximate generalizations of performance. Because of inconsistent Figure 26: Typical data set for step tests, a) step response, b) Nyquist plot and c) L* contrast 61 lighting conditions between different samples we can only make visual comparison of contrast and not quantitative between samples. From charge transferred, within uncertainty there was no great difference in charge transfer between the LiClO4 and EMITFSI samples for the same types of films. This indicates that the difference in ability of small lithium ions and large EMI+ to intercalate is not significant enough to make a difference within our larger sample variation uncertainty. We don’t expect the reduction and oxidation charges to be the same because we are not applying symmetric potentials with respect to the open circuit potential difference between the PEDOT and ITO. Table 5: Step tests for semi-IPN electrolyte cell results, ITO counter electrode (BMIM used to abbreviate for BMIM OSU) Electrolyte, sample|substrate Q (Red/Ox) (mC/cm2) τelec (Red/Ox) (s) (1/e) Imax (Red/Ox) (mA/cm2) τopt (Red/Ox) (s) (90%) DCPD LiClO4/PC BMIM/PEDOT:PSS|glass -0.48/0.62 0.5/1.6 0.34/0.19 8/7 PEDOT/PU|ITO -2.2/1.6 0.9/0.2 0.10/1.6 8/13 DCPD EMITFSI BMIM/PEDOT:PSS|glass -0.33/0.37 0.9/1.3 0.12/0.17 10/7 PEDOT/PU|ITO -1.3/0.95 0.6/0.2 0.8/1.2 7/10 PEDOT/PU|BMIM|glass -1.1/0.81 0.2/0.7 0.37/0.43 16/15 AiBN EMITFSI BMIM/PEDOT:PSS|glass -1.1/0.70 3.3/3.5 0.19/0.09 11/11 PEDOT/PU|ITO -0.78/0.56 1.3/0.2 0.29/0.95 13/8 PEDOT/PU|BMIM|glass -1.8/1.4 4.0/3.2 0.36/0.28 14/14 PEDOT/PU showed larger charge transfer, consistent with it containing more PEDOT. Within uncertainty, the electric switching time constants were observed to be similar for LiClO4 and EMITFSI DCPD membranes, with slightly increased times for AiBN, perhaps indicating a larger charge transfer resistance, potentially due to the heterogeneity affecting the interface and reducing the quality of the electrode electrolyte interface. The electrical reduction and oxidation time constants are not clearly different but differ greatly from the optical time constants. This could be due to the time it takes for the potential difference to propagate, as the first charge to transfer is close to the contact and has a low resistance path, and then the other charge is limited in the speed it can depart by the mobility of counterions and the resistance of the film as the potential front travels across the film. Some of the optical time constants for the EMITFSI DCPD membrane are higher than those for LiClO4, which could be due to the difference in ability for 62 the larger ions to move inside the PEDOT film. The PEDOT/PU on ITO films do not display as much variation. This is potentially because they are on highly conductive ITO that is not changing in conductivity during the reduction and oxidation in the same way that the PEDOT:PSS electrode is. The change can slow down the charge transfer due to varying potential as the PEDOT reduces and accommodates a larger drop in the potential difference, so the whole potential difference is not felt across the electrode-electrolyte interface. The PEDOT/PU feels the same potential across the entire film simultaneously throughout the reaction. In the PEDOT/PU on PEDOT:PSS films it takes longer for the changes to occur, giving further evidence of this. Overall, it appeared that all samples gave somewhat similar performance with ability to reduce and oxidize in under ten seconds, with BMIM OSU/PEDOT:PSS being the fastest and PEDOT/PU on BMIM OSU/PEDOT:PSS being the slowest. There were no clear trends of difference between the electrolytes used, indicating all might be suitable. Table 6: Cells using BMIM OSU/PEDOT:PSS as both electrodes Electrolyte Voltages applied (V) Q (Red/Ox) (mC/cm2) τelec (Red/Ox) (s) (1/e) Imax (Red/Ox) (mA/cm2) τopt (Red/Ox) (s) (90%) DCPD LiClO4/PC -3/0/+1 -1.6/1.4 0.2/1.6 0.64/0.45 7/6 DCPD EMITFSI -3/0/+2 -1.5/1.8 0.2/0.2 1.3/0.79 8/7 AiBN EMITFSI -3/+1 -0.6/0.7 2.5/7.4 0.17/0.06 10/13 Table 6 shows results from testing symmetrical BMIM OSU/PEDOT:PSS on glass cells with three different electrolytes with 6.4±0.4cm2 active area. The test conditions were slightly different, adjusted to for optimal visual performance and cannot be precisely compared. We observe an increase in time constants compared to the ITO counter electrode case, attributed to BMIM OSU/PEDOT:PSS being of higher resistance, taking longer for full potential propagation across the film and for electron and ion transport through the film. Also, both working and counter electrode are undergoing redox reactions so the dual reactions may take longer due to changes in resistance across each electrode. Overall, less charge was transferred for the AiBN membrane, the electrical time constants were higher, and the current peak lower, potentially having a poorer electrode-electrolyte interface interfering with rapid charge transfer. Overall, the optical time constants were similar. The overall result showed that appreciable switching speed and visual contrast could be obtained in <10 s with small voltage steps applied without the need 63 for ITO, validating use of PEDOT:PSS alone as the electrochromic, electrode and ion storage material, and showing each of these electrolytes might be candidates. We also tested the Magenta/polyurethane, discussed in Chapter 2, with the semi-IPN electrolytes, on ITO or BMIM OSU/PEDOT:PSS, briefly shown in Figure 27 and discussed in Appendix A, with the ability to achieve 4 different colour states with the PEDOT and Magenta cells. The purpose of this was to start exploring possibilities of intrinsically stretchable multi-coloured cells. 3.4.2 PVA/H3PO4 3.4.2.1 Mechanical According to the stress strain relationship plotted in Figure 28a, the Young’s modulus was calculated to be 2 MPa, and it appears highly elastic for this range of ~30% strain. At the maximum instrument range of 250% strain as in Figure 28b the electrolyte did not break. Figure 27: Magenta/polyurethane (M) cells using DCPD semi-IPN with EMITFSI, a) M on ITO vs. ITO, b) M on BMIM OSU/PEDOT:PSS (B) vs. ITO, c) M on B vs. B Figure 28: PVA/H3PO4 characterization a) stress strain for 1:1.5 PVA:H3PO4, b) transparent and TiO2 opaque versions, stretch to 250%, c) Nyquist plots for ionic conductivity calculation 64 The PVA mixture cast onto BMIM OSU/PEDOT:PSS on glass excelled in the peel test: the entire PEDOT film came up from the glass to attach to the electrolyte, and still remained conductive. This was thought to be to do with strong electrostatic bonding between the PVA hydroxyl groups and polar groups in the PEDOT and PSS. 3.4.2.2 Electrical For the PVA electrolytes the Nyquist plots are shown in Figure 28c and ionic conductivity is calculated to be 2.6 mS/cm for transparent and 3.1 mS/cm for the white electrolyte, comparable to the previously reported 3.4 mS/cm [106]. It is plausible that TiO2 could cause an improvement in conductivity due to additional ionic transport sites via TiO2/polymer interface according to previous studies, discussed more in Chapter 4. The uncertainties in thickness, area and concentration of components and water content remaining could also explain the difference in conductivity, so more tests would have to be performed to confirm this. 3.4.2.3 Optical In Figure 29b we can see the white PVA/TiO2 electrolyte compared to the transparent one, and they are observed to have visually better transparent and opaque quality than the semi-IPNs. In Figure 29 we observe as-made samples using PVA/H3PO4 transparent and opaque separators on different surfaces on SEBS. Evidently, the transparent version may have poor contrast on dark backgrounds. We also show versions of the transparent opaque cells reducing and oxidizing and observe high optical contrast, discussed in the next chapter. Figure 29: PVA/H3PO4 electrolyte cells a), b) cells using transparent and opaque electrolyte on different surfaces, c) transparent cell switch, 3V d) later opaque cell switch, 1.6 V 65 3.4.2.4 Electrochemical 3.4.2.4.1 Stability of PVA/H3PO4 We tested electrochemical stability of H3PO4(aq) and solid PVA/H3PO4. The known stability window of aqueous H3PO4 due to water hydrolysis is expected to limit the range of voltages that can be applied, so it was important to understand the maximum range. As discussed, this potential window might be expected to widen as the pH decreases, due to the shifting of the hydrogen evolution to a lower potential. This shift was first confirmed by testing the stability range at two different pHs in water. The solid electrolyte stability window was then tested. In Figure 30a we observe the plots from testing aqueous stability for 0.4 M and 5 M H3PO4, and see that for the same working electrode, the reduction onset moves lower by about 100 mV, as expected. This shift might translate to an additional shift to the negative with the solid electrolyte, though this is hard to predict as it is not under standard aqueous conditions. There is still significant current below -0.5 V vs. Ag/AgCl in the low pH electrolyte. In the aqueous case at least, this will lead to unwanted parasitic currents since the PEDOT film showed reduction peaks around -0.7 V vs. Ag/AgCl in LiClO4/PC. We then measured the two electrode stability window of the PVA electrolyte between two carbon black elastomer electrodes, as shown in Figure 30b. We observe that significant hydrogen reduction does not occur until 1 V across the cell. In our eventual choice of voltage operation for our electrochromic cell of 1.6 V, the parasitic reaction is occurring (Chapter 4). As we will see, the parasitics do not prevent colour change. This is likely because the parasitic currents per area are small – about 1 A/cm2 at 1.6 V. However, they may be a major source of degradation, Figure 30: Stability window of H3PO4 electrolytes with cyclic voltammetry (10 mV/s): a) three electrode aqueous H3PO4 using carbon black and ITO working electrodes and a carbon paper counter electrode, b) two electrode tests of carbon black/(Solid PVA/H3PO4)/carbon black cells 66 causing low cyclability. We return to this discussion in Chapter 4 and show that introduction of titanium dioxide into the separator may provide a stabilizing effect. 3.4.2.4.2 Electrochromic step tests Voltage step tests were performed using each PVA electrolyte, with evaluation of suitability based on step response, speed and degree of coloration. In our tests for BMIM OSU/PEDOT:PSS with PVA/H3PO4, we fabricated electrodes directly on a SEBS substrate, and used identical working and counter electrodes. In Figure 31 we observe the optical and electrical patterns for an applied 3 V step potential across an early PVA sample. The associated numbers are presented in Table 7, scaled by the approximate area of 1.6 cm2. 0 V was used to reoxidize. It is noted that the reduction and oxidation charge is highly imbalanced. It would be expected that 3 V would produce high levels of parasitic reactions, as this was before we decided on the step voltage – but parasitic currents are still relatively small at this applied voltage, as is evident by the near return to zero of current after 10 s. Table 7: Electrolyte results for PVA/H3PO4 with BMIM OSU/PEDOT:PSS sample Q (Red/Ox) (mC/cm2) τelec (Red/Ox) (s) Imax (Red/Ox) (mA/cm2) τopt (Red/Ox) (s) -2.4/1.2 0.6/1.9 0.45/0.14 1.9/2.0 It is apparent that the charge transfer per area is comparable to higher levels of the BMIM OSU/PEDOT:PSS with semi-IPN in Table 5 and Table 6. As the films are expected to have similar volume/mass of PEDOT:PSS content due to similar thickness it is apparent that this may be a real effect of more proportion of effective total charge transfer with this electrolyte. This could be due to a high amount of PEDOT able to be Figure 31: a) Step potential on PVA/H3PO4 sample, b) L* contrast from video 67 reduced or oxidized due to lower barrier to charge transfer at the electrode-electrolyte interface or PEDOT overoxidation (above 1.4 V vs. SHE). This could also be a reason the reducing and oxidizing charges are not equal, or that the PEDOT doesn’t fully reoxidize. There was a ~0 V open circuit potential initially such that it cannot be attributed to charge imbalance in the as-cast state. Though these are the first cycles, a similar trend is observed for later cycles so it is not due to charge equilibriation in initial cycles. This could also be due to the low sampling rate (5 Hz) of the potentiostat not capturing the peak current of the fast reaction leading to charge integral inaccuracy. The hydrogen ions or charged cationic phosphate species may be able to penetrate the film better than the EMI+ and Li+ ions. The optical time constants are much less than the semi-IPN cases. We believe that the H3PO4 additionally dopes the PEDOT:PSS, increasing conductivity, as has been previously reported for acids, which would be expected to improve performance [51]. In the semi-IPN cases a ‘front’ of reduction or oxidation moving across the film observed visually indicates a progression of charging across the width, potentially suggesting an electronic rather than ionic limit to the rate. The peak current is comparable to some of the other cases with the semi-IPN, likely from the similar resistance of the electrodes producing a similar initial current for a given voltage step. The overall reaction could be faster due to less total charge transferred (smaller area of ~2.5cm2 vs. 6.4cm2) or better charge transport in the electrode. From the optical image, it is apparent that comparable contrast to that in the semi-IPN is achieved, with faster speed, indicating a lower barrier to charge transfer at the electrolyte-electrode interface. 3.5 Discussion Overall, in this chapter we identified that PEO/NBR semi-IPN was a compatible stretchable electrolyte with the selected electrochromic, electrode and ion storage layer, BMIM OSU/PEDOT:PSS, for electrochemical, electrical and optical characteristics - but was not able to form a strong mechanical interface necessary for building a stretchable cell using our method. We identified PVA/H3PO4 as a good candidate in that, in addition to all the other desired properties and a better electrode-electrolyte interface, it formed a strong mechanical bond required for a stretchable cell. There were some concerns about electrochemical stability, but nevertheless in the next chapter we explore developing the PEDOT and PVA layers into a fully vertically layered cell that operates electrochromically under strain. 68 Chapter 4. Intrinsically stretchable electrochromic cell 4.1 Background In this chapter we build and demonstrate our single pixel intrinsically stretchable electrochromic cell. We base this cell on the stretchable BMIM OSU/PEDOT:PSS film chosen in Chapter 2 for high stretchability, optical contrast and electrical conductivity – used for the electrochromic, electrode, and ion storage layers. We use the PVA/H3PO4 electrolyte chosen in Chapter 3 for high ionic conductivity, stretchability, electrochemical compatibility and strong bonding. A carbon black elastomer composite is used for contacts in order to withstand large strains, and we design, test and implement a setup to take quantitative measurements of electrochromic device performance under strain. We demonstrate contrast and switch speed comparable to other PEDOT electrochromic flexible devices in the literature and show minimal performance degradation under 30% strain. Moderate cycling stability is demonstrated, and we discuss degradation mechanisms. This is a prelude the Chapter 5, where multiple devices are demonstrated as an array. 4.1.1 Base structure As introduced in Chapter 1, the desired electrochromic cell structure requires making all layers stretchable and electrochemically, electrically and mechanically compatible under strain, while maintaining functionality. Below we review our choices to meet each of the component layer requirements given in Chapter 1 in an electrochromic cell, given the stretchability requirement. We add details not previously covered, relevant to optimization of the cell for each layer, such as substrate and counter electrode. 4.1.1.1 Substrate In flexible electrochromic devices, degradation with bending due to poor adhesion of the electrochromic layer is a large issue, such that bonding to the substrate is a very important consideration [40]. This becomes even more important when stretchability is also introduced. We choose SEBS as the transparent elastomer substrate and encapsulant due to its good optical qualities as well as its adhesion to and previous use as support for stretchable PEDOT:PSS films [6]. Plasma treatment of thermoplastic elastomers has been found to improve wettability between 69 materials to be bonded, promote intimate contact, mechanical interlocking and interdiffusion of polymer chains between surfaces, and higher surface energies due to electrostatic interactions and creating more polar functional groups [116]. This is important as SEBS and many other elastomers are initially hydrophobic and PEDOT:PSS water dispersion dewets from the surface, preventing spin-coating of uniform films. We use plasma treatment on SEBS to enable wetting and spin-coating of hydrophilic solutions of PEDOT:PSS and to promote the creation of a mechanical bond [116]. 4.1.1.2 Electrodes and electrochromic layer In Chapter 2 we found our PEDOT:PSS doped layer conductive enough to use as an electrode on its own and we did not need an additional conductive layer. Overall, we confirmed in Chapters 2 and 3 that our choice of PEDOT:PSS composite has the required high contrast, high electrochemical stability and fast electron transfer kinetics to enable high performance. 4.1.1.3 Electrolyte In Chapter 3, we confirmed that our PVA/H3PO4 electrolyte had high stretchability, high ionic conductivity, low electronic conductivity, and the ability to form a good electrochemical contact with the electrochromic layer, enabling high contrast relatively rapid reduction and oxidation among other electrochromic devices. The electrochemical stability was determined stable enough for a few test switches but with concerns about parasitic hydrogen reduction making it unclear what would happen when cycling the device- something addressed in this chapter. 4.1.1.4 Counter electrode/ion storage layer We opted to use PEDOT for the ion storage layer to perform the compensating reduction or oxidation reaction, similar to previous examples [117], [118], which is shown in principle to work in our initial tests in Chapter 3, but it is worth exploring this choice in more depth as it can alter the device stability and limit its contrast. Poor performance attributed to electrochromic polymer stability is often actually due to inadequate charge compensation capabilities of the ion storage layer [115]. Changing the counter electrode can lower operating voltage, remove equilibration time, and improve cycling stability. 70 The ion storage layer at the counter electrode stores charge that is equal to and opposite to the charge at the electrochromic working electrode. In the case of transparent cells, it may also itself be active in the electrochromism. For charge neutrality to be maintained and for charge to flow, when the electrochromic polymer is oxidized at one electrode, a reduction must occur at the counter electrode. Sometimes it is known what the reaction is, while sometimes it is unknown and may even degrade the cell. For example, in a reported flexible passive matrix display based on PEDOT:PSS, carbon was used as a counter electrode with an electrolyte cast from water, and it was believed that water hydrolysis was the counter reaction [41]. If the counter electrode can’t accommodate the extra charge, the current will result in degradation of either the electrode, the substrate or the electrolyte. Alternatively, the counter electrode may dominate the voltage drop across the cell, leading to incomplete reactions at the working electrode. For example, if the counter electrode is effectively behaving as a capacitor, and its capacitance is much lower than that of the working electrode, the counter electrode will see most of the applied voltage. In comparison to high cycling stability electrochromic samples that have performed for hundreds of thousands of electrochemical cycles, in a tested device that exhibited 42% reduction in optical contrast after only 100 cycles, the poor cycling was attributed to uncompensated charge balance between the electrodes [41]. Overall, it is important for us to be aware of how our choice of counter electrode may affect performance and consider in the event of poor results or cycling stability how we might change the counter electrode to optimize the structure. 4.1.1.5 Contacts To form the integrated electrodes it is necessary to build an interconnect that is both chemically stable and mechanically durable under strain. In this chapter, contacting the thin fragile electrochromic material without fracture at the hard contact to soft PEDOT layer interface upon minimal deformation was challenging. This is a common problem in soft electronics termed the ‘hard to soft interconnect’ problem, where differences in modulus at connecting points can lead to concentration of stress and rapid mechanical failure or fracture upon application of small mechanical stress to soft and delicate materials [119]. We had some inconsistent performance with copper contacts including chemical degradation. It has been found that graphite and other carbon-based materials can form an ohmic contact to PEDOT, while some 71 metals form a rectifying contact [120], and we utilized this knowledge along with a carbon-based soft electrodes to form a stable contacts, and address the hard to soft problem. 4.1.2 Performance improvement In this section we present a brief literature review regarding key factors of electrochromic performance- speed and contrast- to give best values for with which to compare our device. 4.1.2.1 Speed The speed of an electrochromic device is determined by the rate of the reduction or oxidation, which can be reaction limited, and is on the order of milliseconds to minutes [121]. PEDOT based flexible displays have switch times reported of 40 ms-20 s, typically around 1-5 s [10], [117], [118], [122], [123]. If the electrolyte or the electrochromic layer have poor ionic conductivity, it can limit switching rate. Many electrochromic devices using ITO in contact with the active layer can take seconds or minutes to change colour. Nanostructuring the electrochromic surface for PEDOT-containing devices by creating nanopores, nanotubes or nanolayers has been shown to allow ions to more easily intercalate, allowing them to reach their redox counter-charge transfer sites faster, showing switching times of under 600 ms - and even as low as <10 ms - on the order for the requirement for fast refresh displays [121], [124]–[126]. We do not take advantage of this, but it offers a potential path for future improvements and would allow us to achieve higher switching speeds. The small amount of previous work on stretchable electrochromics provides some examples of switching times- here we present examples from literature in order to provide a frame of reference for our device’s performance. Among encapsulated devices, one encapsulated device a 4 s switch for unstrained and a 10 s switch for the strained case were reported [48]. A carbon nanotube electrode device using P3HT under 20% strain exhibited <4 s switch time [49]. The only intrinsically stretchable electrochromic material reported, PEDOT/polyurethane, when combined in a lateral electrochromic cell structure with hydrogel electrolyte and external metal contacts showed 20 s switch time, but the full layered device was not encapsulated nor stretchable [5]. Some materials were not encapsulated but immersed in electrolyte and switched. In electrochromic spandex a 400 ms switch time was achieved when the electrode is immersed in 72 electrolyte [50]. The above mentioned intrinsically stretchable PEDOT/PU film showed a 10 s switch time in electrolyte immersion [5]. 4.1.2.2 Contrast PEDOT-based electrochromic devices have achieved high contrast, recorded commonly as transmission, reflectance, or optical contrast change ΔE* dependent on the CIE L*a*b* colour standard, discussed in detail in 4.1.4.3. Contrast ratio (ratio of luminosity of bright to dark) and coloration efficiency (charge per contrast unit) are often used to compare performance. Here we discuss some of the common and best contrasts reported for electrochromics, to which we compare our performance, and we present some considerations for future improvement. capability PEDOT transmissive symmetrical devices have exhibited up to ∆Tmax=18-22% (~600 nm), coloration efficiency up to 390 cm2/C, switch time <3 s, and 10 000 cycles capability [10], [117], [122]. A symmetrical ITO-free PEDOT device reported ∆T=15%, with time constant 16-20 seconds [118]. For an opaque ITO-free device with PEDOT as both electrodes, up to ∆E*=33 contrast was achieved, with a switch time of 1-4 seconds [10], while another with optimized electrode thicknesses exhibited ∆E*=42 with a switch time 41 ms [123], taking advantage of doped flexible PEDOT:PSS for high conductivity and a thicker counter electrode to achieve this. We are limited in our future attempts to implement this by the need to preserve stretchability of the electrodes, such that too thick electrodes or certain dopants will not provide this. Though these are values for PEDOT alone, high performing contrast other electrochromic material performance is summarized below, to consider possibilities of implementing stretchable versions with other materials. Other devices based on thiophene derivatives have reported increased transmission changes based on polymer structural engineering. The Reynolds group reported recently a device based on thiophene derivatives to exhibit up to ∆T=73% [47]. Another flexible device using PEDOT:PSS as electrodes, with Prussian blue and thiophene-based electrochromic layers reported up to ∆T=49% [7]. This indicates that combining our stretchable electrodes, solid electrolyte and substrates with other electrochromic polymers could contribute to much higher contrast stretchable displays than what we report below. We consider this as future work in our conclusion. 73 The only stretchable electrochromic work that reported absolute reflectance change showed that an intrinsically stretchable electrochromic film based on PEDOT exhibited ∆L*=20, ∆T=20% at 570 nm when cycled in solution [5]. In terms of optimizing contrast, a lot can be done by changing the thickness of the electrochromic and counter ion storage layers to ensure highest optical modulation and adequate charge compensation. We do not examine this, but it could be used for further improvements or lower operating voltage. Contrast change will increase with increasing amount of PEDOT until a certain point when the amount of PEDOT will make the electrode dark enough that the contrast change will no longer be as apparent. ITO is the most used counter electrode, but it often cannot provide the maximum contrast as it is a poor charge storage material, and sometimes cannot store enough compensating charge for the electrochromic reaction to go to completion. One work found that if both electrodes were PEDOT:PSS in an opaque cell, the counter electrode had to be 4× thicker in order to obtain full coloration at the working electrode, and also this increased contrast sensitivity, requiring lower voltage for same contrast change [123]. This makes sense, since the thicker counter electrode requires less voltage to provide the same charge storage but takes longer to charge. For future iterations of electrochromics, a polymer has been developed by the Reynolds group as an effective counter electrode since it is colourless, solution processable, high capacitance, n-doped when cast, and easily oxidized with minimal loss of electroactivity [115]. The preceding literature and considerations are considered to understand where our device falls in terms of contrast behaviour, and how we might act to improve it. 4.1.3 Degradation 4.1.3.1 Cycle stability Poor cycle stability is a major problem preventing commercialization of current electrochromic flexible devices. Our primary goal was not to create a high cycling stability but to demonstrate a first intrinsically stretchable device, but because our electrolyte of choice contains H3PO4, known to have a strongly degrading effect on many electrochromics and a narrow potential window, it was important to look at what might reduce cycle life in order to ensure a path towards usable device performance. With some flexible electrochromic devices able to operate for at least 1 million switching cycles without significant degradation [127] there is much 74 optimization of material choice and structure that must go into building a long lasting device towards commercialization. Among stretchable electrochromics, cycle stability is poor still as this is a new area of exploration. A tungsten oxide AgNW based device showed 20% contrast reduction after 100 cycles [48], while the intrinsically stretchable PEDOT/PU film showed 20% degradation after 1200 cycles when operated in electrolyte [5]. Thiophene based polymer P3HT operated in a device under 20% strain showed minimal contrast degradation after ~90 cycles [49]. The main reasons for degradation in all electrochromics are erosion of the electrode, permanent ion incorporation, electromigration, corrosion, cycling induced crystallization, ion incorporation from the substrate, or formation of an insulating layer with cycling [128], [129]. Erosion refers to physical degradation mechanisms, such as gas or ion intercalation creating nanoscale holes, while corrosion refers to chemical parasitic reactions that render material electrically and electrochromically inactive. Degradation mechanisms for electrochromics have mainly been studied in tungsten oxide devices, with many possibilities also being applicable to PEDOT. Increased water or oxygen speeds degradation, thought to be largely due to immobilization of cations reacting with water molecules and irreversible oxidation [40], [130]–[132]. Ion trapping and erosion from intercalation, where large ions produce physical damage to the electrode material while intercalating due to large size, can have severely detrimental effects as well. Ions can also often become permanently trapped and reduce charge transfer with cycling, also causing reduction in cell potential, shift of redox peaks and decrease in switching speed [40], [128], [133]. A tungsten oxide device was cycled using H+, Na+, and Mg2+ counterions and it was found that protons were easiest to embed and escape due to small size, but caused corrosion and easily reduced to hydrogen gas [134]. Sodium ions provided the most physically damaging effect out of these three ions, as they have the largest radius, though its effect was not compared to lithium, whose solvation sphere is larger. Increased potentials as well as asymmetric switching cause faster switching speed or higher contrast but poorer cycle performance, perhaps due to increased ion incorporation and damage [135]. There are also observed changes in crystallization due to ion intercalation, reducing insertion capability [40]. Mesoporous rather than dense films have fared better with switching speed and reversibility by providing easier access to charge transfer sites for ions, enabling lower barriers to desired reactions rather than parasitic degrading ones [136]. 75 PEDOT-specific degradation is highly susceptible to non-ideal thermal, moisture, and high O2 or CO2 concentration conditions, causing accelerated chemical degradation, with possible overoxidation of the backbone sulfur and decomposition of counter ions [137]–[139], such that it is important to minimize these factors. PEDOT has a limited potential window, with overoxidation leading to a loss of electroactivity around 1 V vs. Ag/AgCl [93]. Though PSS is highly acidic, a PEDOT layer was found to degrade in acidic pH<0.5 [140]. To study damage due to ion intercalation and deintercalation, PEDOT was cycled in BMI TFSI and BMI DCA ionic liquids [141]. The smaller DCA anion improved switching speed and exhibited less morphological film degradation afterwards, indicating that PEDOT is also susceptible to erosion. One way to improve cycle life in PEDOT cells is pre-treatment with H2SO4. This is shown to provide microstructure ordering by increasing crystallinity and charge transport, and reducing PSS content [142]. This increased operational stability in aqueous electrolytes, likely due to swelling and deswelling having less of an eroding effect. 4.1.3.2 TiO2 in electrochemical and electrochromic cells We added titanium dioxide particles to the electrolyte for some samples to impart white colour, creating a reflective rather than transmissive device, where only one electrode could be viewed so larger contrast could be obtained. In the transmissive sample our ‘white’ state is the neutral state and does not fully oxidize the PEDOT film as the back will reduce. TiO2 also has been shown to improve other electrochromic cell performance and cycle stability, so here we consider, from past literature, potential influences on electrochemical cell operation. Titanium dioxide added to polymer electrolytes has been found to improve ionic conductivity by providing additional conduction via the interface between the titanium dioxide and polymer [143], [144]. It can also improve the electrode-electrolyte interface resistance, degree of crystallization, chain free volume, and mechanical properties [145]. It improves stability of certain types of electrodes by reducing substrate passivation due to improved mechanical strength, as well as reduced electrolyte penetration, corrosion and erosion [129]. It has been observed to affect the energy levels and bonding at the interface to electrodes, sometimes resulting in easier charge transfer [96]. Added to the tungsten oxide electrochromic cells, TiO2 was shown to prevent crystallization of tungsten by improving tungsten to oxygen bonds, and by reducing ion incorporation [146]. 76 Another group showed the combination improved ion storage capacity, stability and contrast [147]. In [96] evidence of TiO2 as an electron donor was found to widen electrochemical stability of a polyaniline/PEDOT electrochromic device by reducing the valence band energy, causing lowering of oxidation potential and preventing overoxidation, leading to a large improvement in cycle stability. Aside from electrochromic specific tests, when TiO2 was incorporated into PEDOT:PSS composite electrodes for other electrochemical tests, it showed beneficial effects. One PEDOT:PSS/TiO2 composite decreased charge transfer resistance with the electrolyte [148], [149], while evidence has also been found of it causing band bending [150]. It has also been observed to widen potential windows and increase capacitance, thought to be due to faradaic processes related to the Ti4+/Ti3+ pair [151]. Ti(III) and Ti(IV) are a formal redox couple with a potential of approximately 0.1 V vs. SHE, where Ti(III) can be oxidized to Ti(IV) in acidic conditions through proton insertion [152]. In aqueous solution, cathodic peaks for TiO2 have been observed at -0.6 V vs. Ag/AgCl, where Ti(IV) atoms reduce, and -0.1 V, where there is reduction of electroactive surface groups from anodic polarization [153]. With plasma treatment these types of electrodes show increased conductivity, thought to be due to stronger inorganic-polymer bonds from plasma reactions [148]. The electronegative hydroxyl groups can also hydrogen bond with the positive sulfur in the thiophene and weaken PEDOT and PSS interaction, similar to dopant activity. In adding titanium dioxide to our cell, there may be beneficial side effects – as will be shown in section 4.4.3.4 where cycle life improvements are suggested. 4.1.3.3 Pseudoreference electrodes In a solid-state electrochemical device, it is difficult to probe the reactions occurring at each electrode because a conventional setup using a reference electrode is difficult to insert into solid devices and standard conditions are not maintained. As we are interested in exploring parasitic electrochemical reactions contributing to degradation, probing the electrochemical potentials at each electrode during the oxidation and reduction is of great interest. A standard reference is a non-polarizable electrode. In other words, the voltage does not vary with current [59], [154]. A commonly used reference is a silver wire coated in AgCl, surrounded in saturated KCl(aq). In a standard 3 electrode electrochemical setup used to measure potentials of 77 half-cell reactions, the voltage between the standard reference and working electrode is controlled and the counter electrode swings to any voltage required to produce counter reactions to supply the necessary current, while the current is measured to determine when reactions occur. A conventional reference electrode is too large to insert into a <2 mm thick solid-state device and the standard reference potentials are only equilibrated and stable in certain electrolyte solutions and not for solid electrolytes. For nonaqueous and solid state devices, quasi reference or pseudoreference electrodes are used, usually based on close to ideal silver, AgCl coated silver, or platinum wire [154]. These non-equilibrium electrodes can exhibit minimal potential change in bulk solution while performing measurements, though the potential remains unknown, but sometimes may exhibit some shift dependent on electrolyte [59]. To measure the true potential the pseudo reference must be calibrated using a known redox couple under the same conditions, commonly Fc/Fc+, to evaluate quality and stability over time. For electrochromic devices, Ag/Ag+ and Ag/AgCl are commonly used as pseudo reference electrodes [155]–[157]. For the latter, an Ag wire coated in AgCl, the potential can be constant in an analyte that doesn’t vary with time, but varies widely between solutions and with contamination due to lack of filling solution, and the thin AgCl can easily degrade [158]. In various experiments it has been found to be stable for voltammetry measurements for at least 30 minutes [159], [160], such that we decide to attempt to use it to make measurements. These measurements are described in section 4.4.3.6 and Appendix D allowing us to measure half-cell potentials. 4.1.4 Spectrometry setup background To measure the performance of electrochromic devices, usually UV-Vis spectrophotometers or colorimeters are used to quantify optical modulation, reporting the change in transmission, reflectance, or optical contrast E*. Conventional tabletop UV-Vis spectrophotometers such as the Cary 7000 that we employ for some measurements, require mounting a sample on a holder and inserting into the machine for testing. As we wanted to measure optical modulation under strain, we had to design a custom setup. The requirements are discussed in the following sub-sections. We used a fiber optic spectrometer with 3D printed sample mounts to measure optical modulation while the sample was strained using a BOSE Electroforce. We compared our setup measurements to a Cary 7000 UV-Vis spectrophotometer to characterize measurement accuracy. 78 As transmission, reflectance and colour perception of materials vary widely depending on light source spectral power distribution and view and source angle, we had to consider standards to quantitatively compare our performance to other devices in reference to the International Commission on Illumination (CIE), which sets global recommendations for standardized optical measurements [161]. 4.1.4.1 Transmission In these measurements, spectral intensity passing through a sample is measured, with normal incidence light and aligned optics reducing light scatter as recommended by CIE for direct transmitting samples [162]. 4.1.4.2 Reflectance Reflectance measurements are usually performed using a UV-Vis spectrophotometer or colorimeter, where the spectral reflectance is measured under specific illumination conditions and can be converted to colour and contrast. There are two components to reflection. Specular light is reflected at an angle equal to and opposite the incident light, and results in a shiny appearance. Diffuse reflectance is scattered by the sample in all directions and forms the bulk of colour perception. Reflectance measurements can exclude or include the specular component. Appearance depends on the illumination spectrum and directionality, diffusivity, and angle of illumination or measurement. For absolute reflectance measurements, CIE recommends (0º/d) or (d/0º), normal illumination and diffuse collecting or diffuse illumination and normal viewing, respectively, in order to get an average reflectance for samples with potential angular-dependent reflectance. This approach allows comparison of colour consistency without including gloss or texture effects [162]. Source light is obtained from an integrating sphere, whose inner surface is coated in a highly diffusive white coating. It attempts to provide a perfectly diffuse light source for illuminating the sample, eliminating any angular dependent reflectance [162]. CIE D65 is the most common illumination standard, simulating spectral daylight. Some measurements are illumination-independent but some colour quantities are defined according to this standard spectrum. We use a reflectance probe for strained sample measurements, which illuminates and views from the same angle (45º/45º) and allows use in a geometry where a regular colorimeter or 79 spectrophotometer cannot fit; however we expect some differences from integrating sphere measurements that are attributed to angular dependent reflectance. In previous work comparing measurements made with an integrating sphere vs. a reflectance probe, best agreement has been for achromatic samples [163], with agreement within 7% of reflectance for opaque diffuse samples including leaves or photovoltaic modules, but with up to 20% variation for samples with translucent layers on top [164]. Because of the difference in measurements between these types of instruments, we must confirm whether our reflectance probe setup reports similar reflectance to the Cary spectrophotometer with DRA-2500 integrating sphere attachment in order to report accurate absolute reflectance measurements. We find they are similar. We use this as a calibration of the absolute reflectance for our setup and sample type showing that our single angle reflectance produces similar values to conventional diffuse illumination integrating sphere more standard measurements. 4.1.4.3 Colour theory Reflectance and transmission give quantitative measures of optical modulation, but to quantify perceived contrast, one of the standard ways of comparing electrochromic performance, it is necessary to refer to colour theory, accounting for spectral sensitivity of the human eye. A colour space is defined by the range of colours spanned by a set of colour matching functions that specify reference stimuli spectra over the visible range and follow the general rules of a vector space. Each possible colour spectrum is expressed as a linear combination of these three functions, and the intensities are normalized to remove differences in illumination [165]. Figure 32: a) Spectral responsivities of retinal cone cells, b) RGB colour matching functions, c) XYZ colour matching functions, reproduced with permission [165] 80 The most biologically relevant colour space is mapped by the responsivities of our eye cone cells, shown in Figure 32a. The most common colour space is RGB, defined in 1931 and the colour matching functions are shown in Figure 32b. Cameras often report numbers in sRGB, or linear RGB, which is defined referenced to D65 illumination [166]. The XYZ colour space was developed from RGB for convenience: to eliminate negative values in the colour matching functions, match X and Z to the cone response better, and isolate the luminance independent of hue to component Y. The linear transformation from sRGB to XYZ is shown in: [𝑋𝑌𝑍] = [0.4127 0.3586 0.1808 0.2132 0.7172 0.0724 0.0195 0.1197 0.9517] [𝑅𝑠𝑅𝐺𝐵𝐺𝑠𝑅𝐺𝐵𝐵𝑠𝑅𝐺𝐵] ( 7 ) [166]. The coefficients in the linear combination of colour matching functions are known as tristimulus values, through a set of integrals as in 𝑋 = 𝑘 ∫ 𝑅 (𝜆)𝑃(𝜆)𝑥𝑑𝜆, 𝑌 = 𝑘 ∫ 𝑅 (𝜆)𝑃(𝜆)𝑦𝑑𝜆, 𝑍 = 𝑘 ∫ 𝑅 (𝜆)𝑃(𝜆)𝑧𝑑𝜆, 𝑘 = 100 ∫ 𝑃⁄ (𝜆)𝑦(𝜆)𝑑𝜆, ( 8 ) where k normalizes lightness to Y=100 for a white reflectance standard. P is the spectral power distribution of the illumination, x, y, z are the colour matching functions and R is the reflectance spectrum. Equal changes in X, Y, and Z intensity do not correspond to equal differences in visual perception due to the spectral dependence of cone cell response. The L*a*b* space was first developed in 1976 where non-linear transformations are applied to XYZ values to make the colour space more uniform, so the same amount of colour difference ΔE* corresponds more closely to the same perceived degree of contrast. L* corresponds to the uniform lightness while a* and b* set the hue. The definition of this colour space is expressed in Equation set ( 9 ) and ( 10 ) as: 81 𝐿* = 116𝑓(𝑌 𝑌𝑛⁄ ) − 16, 𝑎* = 500[𝑓(𝑋 𝑋𝑛⁄ ) − 𝑓(𝑌 𝑌𝑛⁄ )], 𝑏* = 200[𝑓(𝑌 𝑌𝑛⁄ ) − 𝑓(𝑍 𝑍𝑛⁄ )], and 𝑓(𝑋 𝑋𝑛⁄ ) = (𝑋 𝑋𝑛⁄ )1 3⁄ if 𝑋 𝑋𝑛⁄ > (24 116⁄ )1 3⁄ , 𝑓(𝑋 𝑋𝑛⁄ ) = (841 108⁄ )(𝑋 𝑋𝑛⁄ ) + 16 116⁄ if 𝑋 𝑋𝑛⁄ ⩽ (24 116⁄ )1 3⁄ , ( 9 ) 𝛥𝐸* = [(𝛥𝐿*)2+ (𝛥𝑎*)2+ (𝛥𝑏*)2]1 2⁄, ( 10 ) where Xn, Yn and Zn are normalizing tristimulus values for a perfectly diffusing white reference surface under the illumination conditions, so the colour space is source light independent [167]. We used transmission and reflectance as well as L*a*b* values to describe optical modulation changes. As we are mainly interested in dark-to-light contrast for displays, similar to [5] we used L* to represent our contrast only, as a* and b* do not change greatly and our change in hue is less important to us for our application, though for other applications where the change in hue is larger than change in luminance these values become more important to gauge contrast change. 4.2 Fabrication In this section we cover fabrication of the vertically layered and encapsulated single pixel electrochromic cell and integrated contacts, along with insertion of a pseudoreference electrode. 4.2.1 Materials PEDOT:PSS was obtained as PH1000 dispersion from Heraeus. PVA (Mw 146-186k), styrene-isoprene-styrene (SIS) block copolymer, toluene, BMIM OSU, H3PO4 85 wt% in water, TiO2 anatase, 99% pure 0.5 mm silver wire, and hydroquinone were obtained from Sigma Aldrich and used as received. SEBS Tuftec H1052 was obtained as a sample from AK Elastomer. Carbon black high conductivity grade was obtained from Alfa Aesar. TeflonTM sheet and copper tape were obtained from McMaster Carr. 5 minute phenol-based epoxy resin was obtained from LePage. 5.25 wt% bleach from Clorox, silver epoxy paint from SPI, and VHB 4910 tape from Digikey was obtained. 82 4.2.2 Cell construction The fabrication proceeded is shown in Figure 33. We iterated through several versions of contacts, optimizing for mechanical and chemical stability: copper with silver paint, carbon black directly in contact with the electrolyte, and carbon black under epoxy, the process discussed more in Appendix B. Films were made as in Chapter 2 for 46% BMIM OSU/PEDOT:PSS on SEBS films, while PVA/H3PO4 electrolyte solution was prepared as described in Chapter 3. Carbon black elastomer electrode paste was prepared by mixing SIS with toluene in a 1.1:5 mass ratio overnight, then adding 0.4:1.1 carbon black to SIS wt ratio. The mixture was doctor bladed onto glass using 200 μm thick spacers along the edges of a glass slide, and then dried for 30 min before cutting the resulting film into strips approximately 1.5 cm×0.5 cm and laying them down at the edges of the PEDOT:PSS film with an overlap of ~0.2 cm. This was covered with a glass petri dish and annealed at 150º C for 7 min to adhere the carbon black contacts to the film. The edges where the carbon joined the PEDOT were reinforced with epoxy resin to shield the electrodes from the electrolyte and cured for 12 h. The PVA electrolyte solution was poured over the film and then dried for 48 h in ambient conditions, and then for 24 h in vacuum. The PVA layer in dried form was approximately 0.5-0.8 mm in thickness. For opaque samples, the PVA electrolyte solution was prepared with 15 wt% TiO2 vs. PVA. Each resulting half-cell was 2.54 cm×1.3 cm in size. Two exposed PVA surfaces were activated using a Harrick Basic Plasma Chamber with oxygen or air plasma at 30 W for 4 min, then clamped together for 2 h to plasma bond the layers. For samples for strain testing, the ends were reinforced with epoxy resin cured for 12 h and cut into dog-bone shapes similar to [6] to concentrate stress on the middle strained portion, away from the contacts. Silver paint was used to attach copper tape to the carbon black electrodes. The silver and copper did not contact the electrolyte. Figure 33: a) Fabrication process full cell, b) opaque sample with pseudo reference Ag/AgCl wire inserted 83 4.2.2.1 Pseudoreference electrodes For 3 electrode experiments, a pseudoreference Ag/AgCl electrode was prepared similar to [159], [160], and we inserted it into the cell after a plasma activation step when bonding the two half cells. The electrodes were prepared starting from 0.5 mm diameter silver wire that was sanded using P600 grade sandpaper, rinsed in deionized water, submerged in 5.25 wt% bleach for 20 min until the surface had turned dark grey from the formation of AgCl, then stored in saturated KCl for min 1 h before drying and insertion into the sample. To calibrate the Ag/AgCl pseudo reference electrode, different aqueous solutions of H3PO4 were prepared with 1 mM hydroquinone as an aqueous soluble known redox couple [168]. Additionally, a solid electrolyte for calibration was prepared by immersing a platinum wire, carbon paper counter electrode, standard Ag/AgCl reference electrode, and pseudo reference electrode into a PVA/H3PO4 solution containing 1 mM hydroquinone and evaporating the water as above to leave the solid electrolyte contacting the electrodes. The standard reference still retained its internal filling chloride solution but the interface to the solid electrolyte would no longer be under standard conditions, but we assumed that the Ag/AgCl standard potential would likely be maintained due to the fact that the filling solution still contacted the wire, though there might be some shift due to contamination. 4.2.2.1.1 Spectrometer setup Sample holders were designed iteratively in SolidWorks and then printed using Rigid or Grey material with 25-50 μm accuracy on a Form3 FormLabs 3D printer, with metal nuts used to provide threading for optical component attachment. As in Figure 34a-b, ¼”-20 and 3/8”-24 thread size metal nuts from McMaster Carr were secured using cyanoacrylate glue (these nuts are highlighted blue), which could then be used as threading for insertion of standard ¼”-20 steel bolts used to secure the fixture and the 3/8”-24 thread collimating lenses. The fixtures were designed to be mounted on a 1/2” optical post for compatibility with standard Thorlabs optical components as in Figure 34a. The transmission fixture contained two interlocking pieces with fitting teeth at the top and bottom as shown to ensure no optical misalignment in the horizontal or vertical direction, with a 1.5 mm slot for the sample, as shown in Figure 34a-b. Though printed with the FormLabs to 50 µm precision, due to possible warping of the 3D printed structure observed for some prints using 84 this resin, the misalignment may be closer to 100 µm. BOSE clamps were used to grip the sample at the sides for strain. The final fixtures are shown in Figure 34c-d. For the reflectance fixture shown in Figure 35, similarly the blue highlightd nuts were secured with cyanoacrylate glue and used to provide threading for bolts that could hold the fixture and reflectance probe in place, with the BOSE Electroforce clamps used to grip the sample from the sides. A thumbscrew was able to hold the reflectance probe in place at a specified distance of ~1 cm from the sample in either normal or 45º configuration. The bottom piece could be tightened to hold a sample or reference flat against the measurement port. The internal region of the measurement ports were painted black to avoid unwanted reflections. The assembled structure connected to the reflectance probe is shown in Figure 35c, and shown with the BOSE clamps holding a sample in Figure 35d. Figure 34: a, b) Transmission fixture components, blue nuts are secured with super glue, c) fixture with fiber optic cables, d) fixture with sample mounted on BOSE clamps, e) full optical transmission experimental setup and light path 85 4.3 Characterization 4.3.1 Optical characterization setups We employed 3 main methods to measure the speed and optical contrast of our devices. We used L*a*b* approximations from sRGB videos to derive approximate values of optical contrast initially, subject to large inaccuracy and irreproducibility due to variation in imaging conditions - especially with top-down illumination for reflective samples. Hence, the values were better suited to relative comparison between states of single samples imaged in the same conditions. Some approximate comparison between transmissive samples could be applied due to better consistency of backlit conditions. Quantitative experiments were performed using the custom-built spectrometer setup while under strain, whose results for optical contrast were compared with a standard Cary UV-Vis spectrophotometer measurement. The camera and spectrometer approaches are now described. Figure 35: a, b) Reflectance fixture components, blue nuts are secured with super glue, c) fixture with fiber optic cable and reflectance probe, RS50 white standard, d) fixture with sample mounted on BOSE clamps, e) full reflectance experimental setup and light path 86 4.3.1.1 L*a*b* Measurements from Video We employed a similar method to [5] to obtain L*a*b* values from camera video. We employ the ‘Yamera’ application on an iPhone 6s to manually set consistent ISO, offset, shutter, focus, temperature and bias values. The imaging setup was mounted above or in front of the BOSE Electroforce for optimal capture angle. An iPad displaying white was used as a backlight for transmissive sample videos in absence of a standard white illumination table. For some of the reflective sample tests, the sample was mounted above highly diffusely reflecting TeflonTM to approximate a normalizing white diffusive condition - but due to varying lighting conditions, normalizing between samples was still highly inconsistent. The photo and video from our camera are recorded in sRGB format. For each trial, we selected a square region as shown in Figure 36 and took an average of the sRGB values of all the pixels in the region as well as a section of the white reference background to use for normalization. Some samples moved over the course of the video for which an intensity-based edge-tracking MATLAB script was written to adjust the position of the box, visually monitored to ensure accurate selection area. We first converted sRGB to XYZ according to Equation ( 7 ) [166], [169] using a built-in MATLAB function rgb2xyz() using reference white D65 as per sRGB definition [166], [170], [171]. We attempted to correct brightness according to lighting conditions by normalizing to Yn=100 similar to literature, but this did not account for angular dependent viewing and illumination [162], [167]. To convert to L*a*b* nonlinear visual perceived contrast, we performed another transform according to Equation set ( 9 ), where our white point was specified as our white pixel’s XYZ values, using built in MATLAB function xyz2lab() [165], [171]. Figure 36: Averaging colour data over sample area and reference white area 87 We used these methods to approximate and compare relative changes of contrast between samples, and then measured the best performing samples using the more accurate and consistent spectrometry systems to report device performance. 4.3.1.2 UV-Vis Spectrophotometry For unstrained samples, a Cary 7000 UV-Vis spectrophotometer was used to measure transmission, and with an external diffuse reflectance accessory (DRA) 2500 containing an integrating sphere, reflectance. Transmission was measured by mounting a sample on a solid sample holder in double beam mode baselined against air while connected to an Autolab PG101 potentiostat/galvanostat to apply 1.6 V/ 0 V voltage steps to reduce or oxidize, with measurements made after at least 40 s of voltage application to ensure reaction completion. Reflectance was measured using the DRA 2500 on the Cary 7000, comparing both specular included (SCI) and excluded (SCE) mode. We followed previously established standard procedures in using a matte black absorbing mask to make measurements on samples smaller than the sample port [172]–[174]. An 8 mm diameter hole was cut into cardboard that was then painted using Rust-oleum camouflage matte black paint, found to have visible reflectance <10% for SCI, and <5% for SCE, meeting the low reflectance requirement for a black mask. In dual beam mode, the reference port was covered in diffuse highly reflecting TeflonTM and the sample port with the RS50 reference standard from StellarNet (95% reflectance in visible range) for calibrating the instrument to determine appropriate beam intensity. For sample measurements, the small sample mask described above was placed over the port and measurements of baseline R=100% were taken using the RS50 and an R=0% was taken using the matte black surface. After setting baselines using these two references, shown in Appendix C, the sample was placed over the masked port with external electrical connections to the Autolab, and the measurement was made - like the transmissive samples - with the specular both included and excluded. 4.3.1.3 Fiber optic spectrometer transmission and reflectance setup We created custom setups for measuring the transmission and reflectance of samples while under strain on the BOSE Electroforce strain system as discussed and shown in Figure 34e and Figure 35e. A BLACK-Comet UV-VIS-SR StellarNet fiber optic spectrometer was used, along with collimating lenses, the reflectance probe, the RS50 reference standard, and 600 μm diameter 88 fiber optic cables from StellarNet. A low power 12 V Phillips halogen lamp was used as a transmission light source and a high power 150 W MI-R-150 fiber optic illuminator as a reflectance light source. Two layers of Lee 200 blue CTB (Colour Temperature Blue) filter are added to improve the signal to noise ratio of light in the 400-500 nm spectral region where there was less source intensity. Uncertainties arising from the setup, light sources, sample attachment, and optical adjustments were measured. For sample measurements, integration times of 1 ms (average 10 frames) (transmission) - 100 ms (average 1 frame) (reflectance) were used, adjusted so the highest light intensity peak for the source was approximately 80% of saturation. 4.3.1.4 Transmission As in Figure 34e, collimating lenses were focused and one was mounted in front of the light source and connected to a fiber optic cable, which was connected to a collimating lens in the custom transmission fixture. Another collimating lens captured the light transmitted across the gap and routed it to the spectrometer through another fiber optic. Due to the inaccuracies in printing, misalignment or poor focusing of collimating lenses, and the flexibility of the print material, errors in measurement due to optical misalignment was a consideration. Sources of uncertainty for transmission were quantified by measuring the light source transmission spectrum through different neutral density (ND) filters ND40, ND60, ND80, and quantifying the difference from the Cary 7000 measurements. The fixture was detached and reattached to measure variation. Test pieces of glass and SEBS were measured at different positions and angles to determine the range in values. Optical adjustments to the input collimating lens were made to determine how different angle of source insertion affected the values. Additionally, several fabricated samples were measured in reduced and oxidized states in the Cary 7000 before testing with the fiber optic system to compare the transmission spectra. The final design of the fixture is shown in Figure 34a-d. It can be slid upwards to allow full reference light source transmission for the 100% transmission baseline and then slid down over the sample such that no detachment or realignment of the collimating lenses was needed between the source and sample measurements. The fixture was positioned to have the sample flat against the receiving collimating lens side to ensure normal incidence. Before each new sample measurement, steel shims ~200 μm thick were inserted below the different feet of the fixture to 89 improve alignment while optimizing signal intensity and spectrum shape to match that of a glass reference. 4.3.1.5 Reflectance The reflectance probe holder shown in Figure 35 is similar to commercially available ones from Ocean Insight and StellarNet capable of (0º/0º) or (45º/45º) measurements, but designed to fit around the geometry of our BOSE setup to enable measurement while straining samples. For our measurements, as in Figure 35e a collimating lens was mounted in front of the filtered 150 W halogen light source and routed into the fiber optic cable connected to the reflectance probe. The probe was mounted in the holder at 45º and the bottom piece could be tightened to clamp samples flat onto the measurement port. The diffuse reflected light was reflected back into the probe and routed through fiber optic cable to the spectrometer. To calibrate, reflectance measurements were compared between the Cary 7000 integrating sphere and Comet. These weren’t expected to match exactly due to the different measurement geometries (diffuse vs. single angle) but would be expected to differ by <8% for opaque diffusive samples, as in the previous references. Like with the Cary, the RS50 standard was used for the R=100% baseline, and the light input was blocked for the R=0% baseline. For comparison to ensure accurate reflectance numbers despite the different geometries we only measured samples, first measured in reduced and oxidized states with the Cary 7000 SCI/SCE setup and then with the fiber optic (45º/45º) setup, expected to be similar to SCE, to compare values. Before each new sample, the setup was rebaselined to correct for drift. We directly used Equations ( 8 ), ( 9 ) and ( 10 ) to convert the sample reflectance spectra to XYZ and L*a*b* values independent of illumination spectra. 4.3.2 Electrochemical control/characterization A Solartron 1287 potentiostat galvanostat and Frequency Response Analyzer 1280 were used to input and measure electrical signals during potential steps, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). An Autolab PG101 potentiostat galvanostat was used as an additional channel to measure the opposing half-cell voltage during three electrode measurements. 90 4.3.2.1 Contact design To identify a good contact, we cycle tested different versions of the contacts in prepared samples and looked at resistance, contrast and charge transfer with number of electrochemical cycles under strain to determine which produced the best cycling performance. The cycling was performed with 20 s steps of 0 V to -1.6 V under 30% or 40% strain. 4.3.2.2 Cyclic voltammetry 2 electrode cell measurements Cyclic voltammetry at 10 mV/s was used to cycle through the voltage range to characterize redox potentials and capacitive behaviour. This was used to choose operating parameters for our cell for the step voltage responses. This was plotted over top of optical measurements from video to characterize optical changes with charge transfer and voltage variation. 4.3.2.3 Electrical and optical switching response of cells under strain A BOSE Electroforce Biaxial Dynamic Mechanical Analyzer was used to extend the samples by up to 6 mm (Sample length under strain ~18 mm). Samples were clamped on either side in the BOSE machine with VHB 4910 tape (3M) to eliminate slipping. Alligator clips were attached to the top and bottom electrodes, with two connections to each layer on either side to minimize switching time across the sample. The BOSE was programmed to strain at 10-30 μm/s to increments of 10% separated by 5 min relaxation times before straining by the next incremental 10%. This was done in order to accommodate for the PEDOT film 72 s relaxation time reported by a previous group [6]. Strains are applied up to 30%. For each sample, voltage step current responses were recorded in a two-electrode configuration, typically applied for 20 s at each voltage level at each strain. For transparent samples usually +1.6, 0, -1.6, 0 V were applied in sequence to observe switching in both directions. For opaque samples, bidirectional or unidirectional step voltages were applied to observe the change between 0, -1.6, and +1.6 V. At each strain position, EIS was used to measure cell resistance and capacitance to confirm sample consistency and that good electrical connection was maintained. Early measurements simultaneously recorded the optical response with the camera, as described above, while later measurements simultaneously recorded with the fiber optic spectrometer setup. The resistances across the working electrode and across the counter electrode in the cell were also recorded in some tests. From the recordings, the impedance, 91 optical modulation, maximum current, charge transfer, speed and coloration efficiency were calculated to quantify performance under different strain conditions. 4.3.2.4 Cycle Testing To evaluate the cell stability under strain, the samples that were strained to 30% were potential step cycled while recording electrical current and optical data until a large decrease in contrast was observed. The decline in optical modulation and charge transfer along with changes in the impedance were used to evaluate stability. For earlier tests, drifts over hours in video recording conditions prevented precise quantitative comparison of optical modulation for samples that exhibited very long cycling capability, but this was improved in later measurements through optical isolation and consistent illumination. 4.3.2.5 Cyclic voltammetry 3-electrode measurements To calibrate our pseudo reference electrodes potential (ps-Ag/AgCl) and measure stability, we used hydroquinone due to its solubility in water and known stable peak redox behaviour in varying pH. First, a 3-electrode cyclic voltammetry setup was tested in an aqueous solution with 1 mM hydroquinone and at concentrations of H3PO4 of 0.4 M and 1 M, using both a standard Ag/AgCl and our ps-Ag/AgCl as references. Pt wire was the working electrode and carbon paper was the counter electrode, used to verify operation under acidic aqueous conditions. The test was repeated with the electrodes in the solidified PVA electrolyte, using both the standard Ag/AgCl and the ps-Ag/AgCl that were encased in the solidified polymer. Open circuit potential stability over ~20 min and minimal change in the separation of the hydroquinone redox peaks in the cyclic voltammogram were confirmed using both references. This provides evidence that the ps-Ag/AgCl is able in this electrolyte and voltage window to hold stable potentials for the ~20 min duration of a cyclic voltammetry experiment, though testing at extreme voltages wasn’t performed, as this irreversibly degraded hydroquinone. In attempt to learn more about the chemical reactions occurring in each half cell, 3-electrode measurements were taken with electrochromic cells. Cyclic voltammetric measurements were made at 10 mV/s (-0.9 to 0.9 V) vs. ps-Ag/AgCl, with video recorded concurrently, such that optical data with current and charge could be plotted. In some tests, two channels, one from the Solartron and one from the Autolab, were used to monitor potentials in both half cells. 92 4.4 Results 4.4.1 Fiber optic spectrometer transmission and reflectance comparison For the calibration of the fiber optic setup it was important to characterize the expected uncertainty of the absolute as well as the change in transmission and reflectance, in comparison to standard Cary measurements. For systematic offsets due to setup variation between the two machines, though the absolute T or R value were found to be slightly (~3%) different, the ΔT or ΔR were closer, showing our system is better for measuring change in these values. The uncertainties are reported in Table 8, with additional details given in Appendix C. Table 8: Remaining uncertainties in the transmission and reflectance setups Uncertainty Transmission Reflectance Range (nm) Percent Range (nm) Percent Random (dΔT, dΔR, dT, dR) Light source 400-700 <0.5% 450-700 <1% Sample 400-700 2% 400-700 2% Total (root mean square) 2% 2% Systematic (dT, dR) Avg difference from Cary 450-700 2% 500-650 1% Nonlinear with source 500-675 2% 400-700 2% Total (root mean square) 3% 2% Figure 37: Light source standard spectral variations over 9 minutes with 2 layers Colour Temp. Blue filter, 1 ms integration time, average over 100 frames 93 For each setup, we analyzed the transmission and reflectance uncertainty due to different effects and took the standard deviation as the uncertainty value over a certain spectral range for our final values, reported in Table 8. Spectra and variations over 9 min (1 measurement per min) of our filtered tungsten halogen sources are shown in Figure 37 to be <1% for most of the visual range. We electrochemically cycled a sample with the Cary to determine the sample-dependent variation in contrast between subsequent cycles and found a maximum difference of 2.7% with each subsequent cycle for a transparent sample, from spectra shown in Appendix C. We estimate the sample variation in reflectance to be similar to the transmissive sample, but this could be better confirmed through measurement. Some uncertainties depend on how the sample was initially set up, with the largest effects being adjusting the optics or the BOSE, causing up to 10% difference in transmission or reflectance. Hence, these were not adjusted throughout an experiment. It was found that varying the angle of the sample even within the 1.5 mm gap could have an effect of up to 3% variation. With careful setup ensuring the sample was flat against the detector-side collimating lens this could be eliminated, but still could have an effect if the sample was not perfectly aligned to be flat when stretched. A next iteration of this fixture will likely include a mechanism to ensure sample flatness through each measurement. We also expect uncertainties to show up in measurement that depend on optical properties in different redox states, but this is difficult to measure. For the transmissive samples, this could be an increase in scattering as light is transmitted, and for reflective samples, this could be a change in the angular dependent reflectance that is measured differently between the Cary and Comet systems. We then measured differences in optical spectra between the Cary and Comet systems to better characterize uncertainty. The useful portion of the Comet spectrum was found to be between 500-675 nm as above this wavelength the intensity was too high due to the filter not blocking this part of the spectrum, and below this there was a dropoff in the spectrum into the UV. As this range is a large portion of the visible (400-700 nm) spectrum and the maximum changes in PEDOT optical modulation usually peak around 600 nm, we judged this as adequate to measure optical modulation. The falloff of the signal below 500 nm as the sensitivity decreases is theorized to be due to poor UV transmission through the fiber optic and greater attenuation below 500 nm known for silica fiber optics [175], but this also may be mainly related 94 to the lower spectral intensity below 500 nm or optical misalignment, poor collimating focus, or spectral dependence of scattering by the sample. First, for transmission, the average difference through the neutral density (ND) filters was found to be 2%, as in Figure 38a, between the instruments, while for the experimental sample measured this was closer to 5%, as shown in Figure 38c. This is could be due to the different scattering effect as light is transmitted through the heterogeneous sample, more dependent on internal and surface texture than results from the ND filters. The angle of acceptance for the collimating lenses is only 3º and would be estimated to be misaligned to up to ~300 μm, while the Cary has a larger angle of acceptance and more consistent optical path, meaning less irregularity in optical path and light transmission. For transmission of the reduced and oxidized states of the sample, shown in Figure 38c, though the values are vertically offset, the change in transmission is consistent with the Cary results in Figure 38d. The vertical offset could be explained by increased scattering by the non-perfectly collimated beam due to optics focus and misalignment, and the small capture angle of the collimating lens, compared to the Cary. Figure 38: a) Measurement of neutral density (ND) filters using the Cary and Comet, and light source spectrum, b) difference of transmission between the Cary and Comet, demonstrating linear relation to light source intensity, c) absolute transmission of sample measured with Cary and Comet, d) ΔT calibration against sample measured with Cary 7000 95 In the reflectance we only compare to the sample, and find an average agreement within 2% for reflectance and change in reflectance as shown in Figure 39, possibly due to the fewer optics creating less alignment error, the fact that the sample could be better flattened against the measurement port, and that we were measuring a diffuse reflective signal off of the surface and not a single-angle directional transmission subject to scattering in the sample. We compare Cary SCI/SCE results plotted in Appendix C for a sample in reduced and oxidized states, to find they are the same but just with a 1.5% offset. The SCE measurements are found to match better with the Comet diffuse (45º/45º) measurements, as expected, but the ΔR is the same with or without the specular content. The difference between the Cary and Comet ΔR falls well within the expected difference of up to 8% between the two reflectance measurement methods for diffuse opaque (white reflective electrolyte) samples. Also, as observed in all diagrams, there is an oscillatory behaviour corresponding to the difference in spectral intensity of the source. In Figure 38b and Figure 39c we plot the difference between the Comet and Cary measurements as a function of the Comet measurement source intensity and observe that there is a correlation between source intensity and T or R for the Comet setup. In Table 8 we report the uncertainty due to the nonlinearity to be half of the total observed variation of the T and R values with source. The slopes of the relation were affected by optics components adjustments. This could be related to light focusing collimation quality or optics and sample misalignment. It is known that that there are differences in light transmission, refraction and reflection between the air and the sample, which could also exhibit interference effects due to the translucent nm scale layers and heterogeneity of the sample, and these might make more of a difference in the optics used in the fiber optic setup vs. the Cary setup due to smaller light angle Figure 39: a) Cary SCE and Comet measurement, b) ΔR% between Cary and Comet, c) Rcomet-Rcary vs light power 96 transmitted or received. It might be better to baseline against a transparent elastomer sample of known reflectance. Similar spectrally dependent effects due to materials and optical setups have been reported in comparing integrating sphere to fiber optic probe measurements [176]–[178]. Without more precise optics alignment and calibration it is difficult to know what aspects to improve. Nonetheless, because our uncertainties compared to the Cary measurements are <3% for transmission, reflectance, and change in each, we can still claim accuracy within uncertainty of our results in cases where our total optical T or R signal or change is much larger than the uncertainty, which is true for many measurements in our case range. For the change in optical modulation, the estimated uncertainty is lower, <2% so we can claim more confidence in the change in optical modulation values rather than the absolute numbers, especially for optical modulations as high as 20%, which we see in some opaque samples. 4.4.1.1 L* vs. transmission and reflectance Our early measurements of optical changes were done with a camera, so we also wanted to see how closely these relative values matched with the ΔT and ΔR values, though the L* was not consistent. We scaled them to overlap on the plots to observe relative optical modulation with time for each value. For the transparent sample in Figure 40 ΔL*=10 vs ΔT=12% (600 nm) and in Figure 41 for an opaque sample ΔL*=30 vs. ΔR=18% (600 nm). They almost completely overlap, indicating a linear correlation between ΔL* and ΔT or ΔR even in the absence of a consistent scale between the two values due to lighting variation. Conditions with the opaque Figure 40: a) Transmission of a sample in reduced and oxidized states, b) images of sample in reduced and oxidized states, c) overlay of T and L*/L0* calculated from sRGB video showing approximate correlation 97 sample on the TeflonTM background were found to be slightly more consistent than on other backgrounds. 4.4.2 Contact design Initial copper tape and silver paint contacts did not withstand electrochemical switching due to parasitic reactions and then we moved to carbon black electrodes for soft mechanical properties, ohmic contact to PEDOT and stability tests. The different samples shown in Figure 42 are as follows: S0: 0% strain carbon black/SIS on top of PEDOT in contact with electrolyte, S1: strained to 40% then cycled same design as S0, S2: strained to 30% then cycled rough carbon black/SIS under PEDOT, S3: strained to 30% then cycled smooth carbon black/SIS under PEDOT, S4: strained to 30% then cycled carbon black/SIS with interconnect points encased in epoxy. We measured resistance with strain across each electrode in the cell (top/bottom), decrease in optical contrast with strain, and contrast decrease over electrochemical cycles. For the different samples, resistance across the top and bottom electrodes with strain are given in Figure 42a as well as optical contrast ΔL* with strain in b. The drop in contrast is correlated with reducing the PEDOT density per area when straining as well as the increase in resistance in reducing the voltage drop across the cell perhaps reducing total charge transfer – though the charge transfer was found to be comparable in most measured cases. We observe that resistance for the best samples only increases 100-200 Ω with 30% strain. This is not the same as previous results reported for this composite in [6], which shows initial increase in conductivity with strain, but this could be due to the different conditions such as the contacting PVA electrolyte and earlier failure in the contacts due to localized stress at the BOSE clamp points. We observe similar increase in resistance with strain for all samples. The S4 epoxy and S3 smooth samples Figure 41: a) Reflectance of a sample in reduced, 0 V and fully oxidized states, b) images of sample in these 3 charge states, c) luminosity of sample from sRGB video with white reference of these three states, d) overlay of R and L*/L0* calculated from sRGB different video when switching -1.6 to +1.6 V 98 appear most stable in resistance, and it appears S3 decreases in contrast with strain faster than the others, with epoxy being the slowest. All samples were strained at similar rates to Wang et al. [6] of ~10%/min to ensure time for the relaxation by the PEDOT:PSS films. For Figure 42c, the contrast versus electrochemical cycles, the cases are not exactly comparable- for S0 at 0% strain the cycle time was only 10 s, while for the others it was 20 s. Also, the sample in which carbon black/SIS was on top of the PEDOT and in contact with both the electrolyte (bridge sample) was strained to 40% while the others were strained to 30%, and all the samples had different PEDOT layer thicknesses, leading to different contrasts. We were mainly looking for any clear trends showing certain contacts were unsuitable. Some samples were cycled for shorter cycle numbers due to mechanical failure or slipping in the mount of the BOSE, so only the first 15 cycles are shown. Overall, the bridge sample S1 appeared to decline the fastest, but the other samples were similar- the S4 epoxy did not drop off as quickly as the others. The contrast is expected to drop with cycling due to increased PEDOT and contact resistance from parasitic reactions damaging the active layers, reducing total charge transfer. Previously it had been observed that driving the sample at voltages of 3 V produced large bubbles in the electrolyte around the carbon black, removing electrical contact, which was attributed to parasitic hydrogen gas formation, after which the sample could no longer be switched. It was predicted that the carbon black contacting the electrolyte might accelerate degradation due to gas formation, and that separating it from the electrolyte with epoxy might reduce this parasitic reaction. From Chapter 3 the PVA electrolyte exhibited low level but still possibly significant parasitic reaction currents at 1.6 V across the cell, which over time could cause delamination of the carbon black to PEDOT electrodes due to excess gas formation. It is Figure 42: Behaviour under strain of different iterations of contacts: a) resistance, b) contrast vs strain, c) contrast with cycles different for samples. 99 expected that this parasitic reaction will also occur on the PEDOT but will compete with the PEDOT oxidation and reduction and may not be as significant. We chose to proceed with the epoxy contacts as it was shown there was no huge difference in resistance, contrast or mechanical durability between this and the other samples and there might be reduced chance of rapid electrode degradation due to parasitic reactions. The implications of drop in performance with cycling mean in the current form the device is electrochemically unstable such that it is not suited to long cycle lifetime applications, but in this state might still be useful for short term use patches. The cycling stability might be improved using one of the previously mentioned tactics such as using a different electrolyte or optimizing switching voltages and electrode thicknesses. 4.4.3 Electrochemical 4.4.3.1 Step responses electrical, optical, under strain Preliminary cyclic voltammetry measurements at 10 mV/s were taken simultaneously with video for a transparent sample, from which L* values were obtained, as shown in Figure 43a-b. These results were used to select an operating voltage for the cell switching step functions. It was observed that the maximum contrast reached an approximate peak or plateau at an average of 1.6 V across the cell for bidirectional switching, close to the maximum current, and then the contrast flattened out prior to reversing voltage direction. Accounting for contrast, speed and parasitic reaction considerations, 1.6 V was chosen for the step potential amplitude as we wanted maximum contrast while minimizing parasitic reactions from operating at too high voltage. Figure 43: a) Cyclic voltammogram to select Vstep, b) I, V and L* with time 100 In terms of electrical performance, resistances as low as <300 Ω across the top or bottom electrodes were observed. EIS measured resistances across the cell were as low as 150 Ω from Nyquist plots at high frequency as in typical sample plots shown in Figure 44, with increase as low as 20-50 Ω with 30% strain. As each of the working and counter electrode is connected at both ends to the potentiostat, this translates to an increase in each electrode resistance of 40-100 Ω with 30% strain over the 2.54 cm length. The lower resistance electrode samples had faster switching due to the higher conductivity through the electrode and contact and smaller voltage drop in the electrode leading to larger division of voltage across the electrode-electrolyte interface driving faster reactions. In many cases a voltage ‘front’ could be observed moving across the samples from either side, with a wave of colour change approaching the center of the sample. This suggests that electronic resistance of the PEDOT itself was limiting charging rate. Also from the Nyquist plots we can predict the cell capacitance from the capacitive reactance at 0.05 Hz. The capacitive reactance X=3300 Ω=1/jωC, C= 0.001 F. Assuming total thickness of the two PEDOT:PSS cell films of 400 nm and active area 1 cm×2.5 cm, this gives a capacitance of 10 F/cm3. Since we expect our film to be close to 50% BMIM OSU, assuming a relatively equal densities, we might expect that the capacitance of the PEDOT:PSS alone is close to 20 F/cm3 though the uncertainty is large as we do not know the actual remaining content of BMIM OSU and PSS after processing or the exact thickness of these particular films. This is on the order of other reported values for volumetric capacitance of PEDOT:PSS of 12-113 F/cm3 [65], [142], [179]. This indicates a notable proportion of the PEDOT can be reduced or oxidized, but could be in the lower range due to some staying highly oxidized from cationic doping and high Figure 44: Nyquist plots comparing typical transparent and opaque cells for frequency 0.05 Hz-0.8 MHz 101 PSS and ionic liquid content. If we were comparing the amount of PEDOT alone we would expect the capacitance to be much higher. 4.4.3.2 Transmission Our first contrast with strain measurements used video L* values. An example is shown in Figure 45, showing the decreased ΔL* with increase in strain, and similar but slightly increased time to change, as well as higher starting values of L* with increased strain, due to increased transparency with strain. Under 30% strain, the sample could still provide high contrast. We measured the transmission change in the 0% strain state with the Cary and the Comet system. As found previously, the systems had similar ΔT values but ~5% lower T values from the Comet. Quantitative transmission values and other results under strain are shown in Figure 46a-c. In the presented sample for which measurements were taken under strain, under 0% strain ΔTmax =8.6%. The best samples showed ΔTmax=15%, though these were not tested under strain. Upon 30% strain a shift in T in the oxidized samples by approximately +8% was observed. ΔTmax=8.5% at 570 nm, approximately the same within uncertainty to the unstrained value. We observe from plot c, showing optical contrast with time for λmax=570 nm, that the response is almost identical in range and speed to the unstrained case, despite the vertical shift in transmission. This indicates minimal loss in ΔT performance despite decreasing the electrochromic material area density and straining the cell. This demonstrates that the electrochromic performance does not exhibit significant degradation under 30% strain. We also observe the electrical and optical performance in Figure 46c in the two strain states and confirm Figure 45: Transparent sample a) visual contrast with strain, b) L* video values with strain 102 they are almost identical in time constant and current behaviour, indicating very little loss in performance with strain. Table 9 summarizes the values for this sample and calculated quantities at Tmax. Electrical time constant is calculated as the time for the current to reach 1/e of the original, while optical time constant according to definition is calculated where 90% of the contrast change has occurred [40]. Optical contrast L* is calculated according to Equations ( 8 ) and ( 9 ). Coloration efficiency (CE) is calculated using either ΔT or ΔR as indicated and integrating the area under the current charging curves, to acquire charge transferred, using the equation from [38], 𝐶𝐸 =∆𝑂𝐷𝑄𝑑 , ∆𝑂𝐷 = ∆𝐴 = 𝑙𝑜𝑔𝑇𝑐𝑇𝑏= 𝑙𝑜𝑔𝑅𝑐𝑅𝑏, ( 11 ) where ΔOD is change in optical density, ΔA is absorbance change, Tc, Rc, are the transmission or reflectance in the coloured and Tb, Rb in the bleached states, and Qd is the charge transfer per area. Calculations are made using 90% transmission change and 1 cm×2.54 cm typical dimensions. Figure 46: Comparing single cell results under 0% and 30% strain: a) Transmission, b) ΔT, c) ΔTmax and current, d) reflectance, e) ΔR, f) ΔRmax and current 103 For our sample, we observe similar ΔT values but lower ΔL* values under strain due to the nonlinear relationship between lightness Y and the luminosity shown in Equation ( 9 ). Under strain the sample is lighter and similar change in transmission is not as apparent to the human eye. For this sample the optical contrast was poor, but other transparent samples showed values up to ΔL*=8.4. To obtain optimal contrast, the thicknesses of the electrodes must be optimized, though this may affect the electrode stretchability and speed or response. The time constants are comparable to some conventional flexible electrochromics and lower than most stretchable electrochromic devices that operate without electrolyte immersion and metal electrodes. The coloration efficiency for the transparent sample is lower than the best literature values of 300-500 cm2/C but within the moderately high range of ~100 cm2/C. As discussed, there is room for improvement by increasing electrode thickness, for example. 4.4.3.3 Reflectance We looked at similar measurements for reflectance. Contrast with strain for a particular sample, as well as the optical modulation ΔL* with time are shown in Figure 47 from a video. Once again, the stretched samples are lighter. No significant change in contrast with strain is visually evident. Measured L* values do not follow the same trend in monotonic decrease in modulation with strain, but this is attributed to the nonuniform lighting conditions changing the angle of light reflection when straining, making the uncertainty due to lighting change larger than the incremental change in luminosity, as the previous transmission sample was backlit and less subject to different reflection angles of light. It would be expected to exhibit a similar trend under uniform lighting conditions. Though these optical modulation values are not precisely quantitative due to varying light conditions- the 10% value dips below the 0% value for example - they can provide the optical time constants. Figure 47: Opaque sample a) visual contrast at different strains, b) L* contrast with strain 104 In subsequent more quantitative reflectance measurements using the Comet spectrometer, results are plotted in Figure 46d-f of contrast, reflectance change and current and best values are summarized in Table 9. Our contrast is in the range of other opaque PEDOT based electrochromic flexible devices in literature, indicating no decrease in this metric by adding stretchability. The coloration efficiency is on par with mid-range values from literature for other electrochromic devices, indicating efficient use of charge to change the contrast. This effective use of charge helps reduce power usage for devices, It is higher than many flexible electrochromic devices (<110 cm2/C) [40] and slightly less that the highest performing PEDOT-based flexible devices (300-500 cm2/C) [7]. It could be further improved through optimizing layer thicknesses for best contrast with amount of charge transfer. Table 9: Best performance quantitatively tested samples (reflectance time constants for faster sample) Transparent Opaque Typical values (PEDOT devices) 0% strain 30% strain 0% strain 30% strain Baseline T%, R% 31.3% (570 nm) 40.7% (570 nm) 31.4% (600 nm) 35.8% (600 nm) Optical contrast (Δ%T, Δ%R) 8.6±5% (570 nm) 8.5±5% (570 nm) 19.6% (600 nm) 18.8% (600 nm) ΔT=15-22% [118], [122] Optical contrast (ΔL*) 6.6 5.5 15.4 18.4 19-40 [10], [123] Coloration efficiency 109±10% cm2/C (ΔTmax) 77±10% cm2/C (ΔTmax) 194±10% cm2/C (ΔRmax) 155±10% cm2/C (ΔRmax) 100-400 cm2/C [118], [122] Optical time constant (red/ox) (90%) 2.9/1.4 s 2.9/1.4 s 1.9/1.0 (best) 0.9/0.9 (best) 50 ms- 30 s [118], [122], [123] Electrical time constant (1/e) 0.68/1.02 s 0.84/1.28 s 0.32/0.32 (best) 0.57/0.41 (best) We observe a small decrease (2-3%) in ΔR with strain, as seen in Figure 46, but this results in an increase in observed contrast due to the shift in absolute reflectance with strain and the Y vs. L* nonlinear relationship. This sample did not perform as quickly when strained, due to the increase in cell resistance upon stretch. We report in final optical and electrical time constants obtained from the sample in Figure 47 as best speed values achieved for a similar sample in Table 9. The capacity for charge storage was similar in these tested samples, such that the 105 difference in RC time constant for this sample was expected to be due to lower contact or PEDOT layer resistance from processing condition variation rather than lower charge capacity. Overall, the performance in terms of contrast change and coloration efficiency is much better in the reflectance sample than in the transparent sample. The contrast change is influenced by changes at all spectral wavelengths and these are larger over the spectrum for the reflective sample. This could potentially be shown to be different in improving the performance of a transparent sample, but will always lack the ability to operate from full oxidation at 1.6 to full reduction at -1.6 due to the fact that the counter electrode PEDOT layer must remain bleached in the oxidized state. It is possible the layer thicknesses might need to be different in the transparent and opaque samples to optimize the contrast. For best performing speed of samples in optical and electrical constants reported, the speeds for the best samples are similar, of around 1 second. Since the electrical time constant is calculated according to the time to go to 1/e of initial current and the optical time constant to 90% of total change, it would be expected that the optical time constant would be longer. If they are both estimated to the same percentage the values are closer though the optical time constants are still always longer by a few hundred milliseconds, likely due to the time needed for charge propagation through the PEDOT layer to the contacts, limited by resistance. Some of the opaque samples showed marginally improved times especially in electrical response but it is unclear if this is due to the opaque sample electrical influence of the TiO2 or due to variation among samples. Overall, these switch times fell on average in the lower and faster range of similar PEDOT flexible devices. 4.4.3.4 Cycle testing In this section we test cycling stability of our samples. We state our results from each test and then speculate on sources of variability between samples and experiments. Overall, the mechanisms that determine cyclability are unclear, but it is possible that titanium dioxide may have a stabilizing effect on the electrochemical degradation, which is investigated in the next section with a three-electrode setup. First, to try to determine if electrochemical cycling or strain has the larger effect on cell degradation, we measured the impedance of a transparent sample, then strained by 10%, measured impedance again, then performed an electrochemical step cycle, and then measured 106 impedance. We repeated this for each strain increment up to 30% as shown in Figure 48. It is observed that the strain had almost no effect on the cell resistance as the curves overlap, but after cycling there was an increase in the cell resistance every time pointing to a change in the cell electrical properties. We then recorded videos of electrochemical step cycling under 30% strain to observe the optical modulation cycle life for each type of sample. In Figure 49 we observe the results of Figure 49: Transparent sample cycle performance under 30% strain, a) contrast at different cycle numbers, b) contrast over ~90 cycles, c) impedance spectroscopy, d) images Figure 48: Impedance of a transparent cell: effects of strain and electrochemical cycling. Left: impedance at strains between 0% and 30%, before and after electrochemical cycles, Right: close-up around real axis showing effect on internal resistance of strain and cycling 107 electrochemically cycling a transparent sample, held at 30% strain, for ~100 step potential cycles, applying 0 to 1.6 V. Figure 50 shows the cycling response of an opaque sample for ~60 cycles using the same voltage range. For both samples, there was a gradual decrease in contrast and increase in time constant. For the transparent sample at cycle 40 there was 90% retention of contrast, while at cycle 80 we observed 70% retention of the initial value, but then the contrast suddenly and drastically decreased. For the opaque sample pictured here we observed 90% retention after 44 cycles before a sudden drop off. From the Nyquist plots it is seen that resistance and the diffusion contribution, characterized by a 45º slope in the plot, drastically increased with cycling for both samples. After the cycling the samples could not be appreciably switched in the allotted time. It is possible that the contrasts could be larger if the cycle time period was increased to allow more time to charge, given the increase in cell internal resistance and suggested change in ion diffusion mechanisms. With the transparent sample there was an upwards drift of the baseline contrast, indicating a possible shift in the equilibrium condition of the relative charge states of the two electrodes as the cycling went on. There was also some drift for the opaque sample (Figure 50b), which was not observed for the cycling towards higher luminosity, as the other electrode was not visible. Much higher cyclability has been demonstrated in one opaque sample, with the same sample preparation. In Figure 51 we observe the cyclability of an opaque sample at 30% strain, based on Figure 50: Example cycling performance of opaque sample under 30% strain, a) contrast at different cycle numbers, b) contrast over ~60 cycles, c) impedance spectroscopy, d) images 108 video data and applied voltage between 0 and 1.6 V across the cell, over a period of 500 cycles of 20 s per step over ~4.5 h. As in Figure 51a, the ΔL* value starts around 23 and decreases over the cycling to 15. The first 10 cycles, EIS plots after cycling, and video frames at different cycles are shown in Figure 51b-d. It is observed that, unlike the samples that degraded in cycling, even after 500 cycles the EIS resistance and capacitance of the samples were still similar to the first cycle. The lighting conditions were not perfectly consistent, leading to drift in the baseline over time as in Figure 51e, where different colours correspond to sequential videos between which the camera was stopped and started again. Small adjustments due to movement of the camera while pressing stop and start may also contribute to the differences. In Figure 51f we shifted the baseline of each run to be 0 to show only the optical change per cycle in attempt to compare ΔL* only. Between the yellow and purple runs, a positional adjustment of the BOSE due to a mechanical issue slightly increased the strain, reducing PEDOT per area, and changed the sample view angle, causing a sudden decrease in the modulation. The response nonetheless exhibits high contrast after cycling, with the raw values from the data showing a 64% contrast retention after 500 cycles. Visually the optical contrast is still high, as seen in Figure 51d. This sample also had Figure 51: Cycle performance of opaque sample 500 cycles, a) contrast at different cycles, b) first 10 cycles, c) EIS plot, d) photos at different cycles, e, f) show original and vertically shifted contrasts over 500 cycles adjusted so oxidized L*=0, with different colours corresponding to sequential video files 109 a relatively short time constant and moderate switching speed even after long cycle time, only slowing down from 1 to ~10 s over the cycling. From the electrochemical cycling results (Figure 49 and Figure 50), the main source of degradation in early cycles is not the strain but the electrochemical cycling. Ion motion during deintercalation and intercalation in the electrode could cause structural changes and erosion of the PEDOT electrode. We expect to see parasitic reactions from hydrogen reduction since we are operating at 1.6 V, which is larger than the ~1.5 V voltage window of our electrolyte. Gas micro bubbles could cause delamination of the electrodes from the contacts. There could be some charged species incorporation causing permanent electrode changes and loss of capacity, but this has not been studied in detail for protonic conductors. Residual water in the PVA hygroscopic electrolyte could contribute. There may also be PEDOT overoxidation occurring that could contribute to degradation over time. It is also possible we get some migration of the BMIM and OSU ions in the electrodes that may contribute to degradation though it is difficult to predict or measure what effect this might have. All of these could affect the electrical properties and charge storage capability of our cell and cause degradation of the electrode, electrolyte or electrode-electrolyte interface. For the increase in time constant, since the electrical switching time would be expected to be dependent on the RC time constant, we expect that if degradation occurs to increase resistance, we would anticipate this to also increase time constant. For long cycle times, such as in Figure 49 and Figure 50, often a sudden decrease in contrast occurs. This perhaps indicates some critical point of degradation, perhaps of the film or contact, at which positive feedback accelerates it. We observe that the cycle life was highest for the TiO2 sample in Figure 51. The average performance would have to be confirmed by doing many repeat tests, but it suggests that the opaque cells could be inherently more stable. We can speculate on whether the titanium dioxide presence could possibly be acting on the electrochemical cell materials to stabilize the cycle life and slow degradation of the opaque over the transparent cells. The TiO2 is only in the electrolyte, but due to reasons discussed in sections 4.1.3.2 and 4.3.2.5 it could stabilize the electrode physical structure or electrode-electrolyte interface - shown to improve cycle life [129], [145], [146] and to lower the energy barrier to charge transfer at the electrode-electrolyte interface [96], [148]–[150]. The titanium dioxide could also introduce electrochemical effects that might lead to higher stability due to redox activity related to the Ti4+/Ti3+ pair [151] or capacitive storage, as 110 discussed in section 4.1.3.2, though we would expect this more if it were integrated into the electrodes. Of course, the difference in cycle life could be due to other factors. The fact that there is no great observed difference in cell resistance or capacitance for cells at zero bias gives no evidence of the titanium dioxide having a special effect. It could be simply from sample to sample variation or from asymmetry in electrode thickness creating uneven voltage division across the cell and having some stabilizing mechanism. In attempt to explore any differences in reactions occurring at different biases, we compare the half-cell voltages in a 3-electrode arrangement, as discussed in the next section. 4.4.3.5 Cyclic voltammetry pseudo reference electrode validation In this section we used a pseudoreference electrode inserted into the solid-state electrochromic cell to study half-cell reactions and charge state of the working and counter electrodes in a geometrical situation where a conventional reference electrode would not fit. We used this to probe whether there are electrochemical differences in the cells using PVA/H3PO4 or PVA/H3PO4/TiO2 that might lead to added cycling stability. In order to use a pseudoreference electrode with known potential, it must be first calibrated against a standard reference electrode to ensure stable and measured potential [59]. It is expected that the voltage, though stable, will not be the same as the standard reference as it is not in an equilibrium with the solution, so the potential must be measured prior to using it as a reference. We used hydroquinone as a known redox couple to compare cyclic voltammograms between the pseudoreference Ag/AgCl (ps-Ag/AgCl) and a standard Ag/AgCl reference electrode in Figure 52: Cyclic voltammetry comparing ps-Ag/AgCl with standard Ag/AgCl, tested in a) aqueous H3PO4, b) solid PVA/H3PO4 111 Figure 52 to determine whether or not the peak separation shifts, which would indicate that the ps-Ag/AgCl does not hold a stable potential, and what the offset from the standard Ag/AgCl is, to determine what stable potential it holds. In aqueous acid shown in Appendix D Figure 71, as the pH of the H3PO4 test solution decreased, the redox peaks of the hydroquinone with the standard Ag/AgCl and the ps-Ag/AgCl grew closer in potential to one another. For 1 M H3PO4 there was a ~100 mV shift between the peaks. When we tested our solid electrolyte, shown in Figure 52 we observed a ~15 mV shift of the pseudoreference, otherwise the curves are almost identical, indicating that according to these tests, the pseudoreference in this window holds a stable potential and behaves comparably to the standard Ag/AgCl electrode. The shift could be due to a Donnan potential across the standard Ag/AgCl salt bridge due to the ion gradient between concentrated NaCl in the reference electrode and the protons outside. We could not test outside this potential range as it caused the irreversible reduction of quinone species, so it is possible that the stability could vary outside this range. In our first samples with the ps-Ag/AgCl inside the cell we ran tests on each half electrode and observed the curves were almost identical, with no indication of drift beyond the validation window. Based on this we assumed the electrode was stable enough to proceed with our characterizations of the electrochemical reactions seen at each electrode relative to ps-Ag/AgCl. We did not observe drift >20 mV in values over the course of our experiments, which lasted a maximum of 30 minutes. 4.4.3.6 Cyclic voltammetry optical, 2 electrode and 3 electrode cell measurements In this section we discuss how we used a pseudoreference Ag/AgCl electrode inserted into the solid electrolyte of our cells to investigate electrochemical behaviour at each half cell of the working and counter electrode and how it may affect our sample’s performance and cyclability. This has been used to explore charge state of the electrodes in a battery [180] but few electrochromic studies have incorporated this, and none in a solid electrolyte. Overall, it is unclear what are the origins of the prolonged life, but this method is able to show some differences in voltage distributions and currents in thin solid-state electrochemical cells that may lead to insight for performance enhancement. 112 4.4.3.6.1 Two-electrode For two samples of similar real and imaginary impedance, voltage was applied in a 2-electrode configuration between the top and bottom electrodes of a transparent and opaque cell while separately the voltage between the top electrode and inserted pseudoreference Ag/AgCl (ps-Ag/AgCl) was monitored. In Figure 53a we observe the cyclic voltammograms produced as voltage was swept across the cells for the first cycle. There was no clear difference between the curves for transparent and opaque samples in terms of peaks or positions- but the opaque cell showed higher current in one voltage polarity than the other. Some variation in response is expected between samples and between half-cells due to process variations that may, for example, result in different thicknesses of PEDOT. In the same scans, another potentiostat channel was used to measure voltage between the top electrode and the ps-Ag/AgCl electrode, and this was subtracted from the full cell voltage to obtain the voltage on the opposing half-cell. This provides the voltage vs. ps-Ag/AgCl in each half cell occurring as we apply the full potential across the cell, giving clues as to what reactions are occurring. The result is shown in Figure 53b-c, where the bottom electrode is negative, shown flipped in polarity to illustrate the subtle differences in voltage from the top electrode. Concurrently a video was taken and the relative L* values plotted along the same time axis. We observed that as the voltage linearly increased, the division across each side of the cell rose at a similar rate until approaching the initiation of the PEDOT reduction reaction around 0.7 Figure 53: a) Cyclic voltammogram 2-electrode (10 mV/s), b), c) 3-electrode measurements voltage division across transparent and opaque half-cells vs.ps-Ag/AgCl, (bottom electrode potential is negative, shown flipped in polarity to illustrate difference from top voltage) 113 V across the cell. In the transparent cell, at this point the voltage of the top oxidizing electrode reached a plateau, often indicative of a faradaic process (possibly some PEDOT overoxidation) and increase in effective capacitance, while the voltage across the other electrode increased, as shown in Figure 54 also. In Figure 53b at the peak of 1.8 V across the cell there was -1.0 V across the reducing electrode and +0.8 V across the oxidizing electrode vs. ps-Ag/AgCl. Because I=CdV/dt, the side with the redox peak has a large C so voltage remains constant while the C is effectively small on the other side. This means that the voltage difference from ps-Ag/AgCl increased over the PEDOT as it reduced. For the opaque sample, the behaviour was similar but at the peak current, the voltage was much more evenly divided across the cell, with approximately 0.9 V across each electrode vs. psg/AgCl. It can be seen from the current curves that the current in this cell is highly asymmetrical. Reconsidering the electrolyte exhibiting parasitic electrochemical reactions above ~1.4 V across a symmetrical cell in Figure 30b attributed to hydrogen reduction, it would be expected that the transparent sample, with more uneven voltage division across the cell and lower voltage vs. Ag/AgCl on the reducing electrode, would experience more parasitic hydrogen reduction. In Figure 30a we see for the aqueous electrolyte it breaks down at high voltages vs. Ag/AgCl, but only ~-0.4 V on the negative side, and one can imagine that the parasitic currents would steeply rise outside this range. We observe that our electrodes do go outside this range, creating strong potential for degradation of the electrolyte. The compensating reaction at the oxidizing electrode might be the overoxidation of PEDOT, as we are close to the range where we might expect overoxidation to begin and the PEDOT is already heavily oxidized. Further evidence of this is seen in the large peak above 0.7 V vs. Ag/AgCl in Figure 54 in three electrode configuration that causes subsequent loss of electroactivity. If so, we might expect the cells to experience faster degradation from electroactivity loss. It’s possible that this asymmetry could play a role in increasing cycling stability. Mechanisms are discussed below. It is also possible that lifetimes could be increased by slightly reducing the switching voltage down from 1.6 V to reduce parasitics and overoxidation but at the expense of lower contrast. Possible reasons for asymmetries were considered. Though voltage division was more symmetric in the opaque cell, charge transfer was more asymmetric. This could be due to slightly different amounts of PEDOT in each electrode, or reduction in electroactivity after the first half 114 CV cycle leading to reduced charge transfer in the second half of the CV cycle. The amount of TiO2 was spread equally through the electrolyte so was not expected to introduce additional asymmetry. If the asymmetry is due to geometry, our method could assist in optimization of electrode thicknesses by permitting the testing of the half-cell potentials to find out how voltage is distributed across each electrode. The other possibility is that the titanium dioxide presence in the electrolyte and at the electrode-electrolyte interfaces affects the behavior, though the earlier sample in Figure 50 did not show the same improved performance. As discussed in 4.1.3.2, it has been shown previously in electrochemical devices to affect the charge state. Mechanisms include additional charge storage due to TiO2 double layer capacitance or redox mechanisms, change in energy required for electron and ion transfer at the electrode-electrolyte interface, change in inorganic-organic bond strengths at the interface, and improving ion conduction through electrolytes by providing additional pathways along the particle surface. We might predict TiO2 along the surface of the PEDOT on either side of the cell could contribute to additional ion storage capability due to capacitive or redox mechanisms. However, from our EIS scans comparing these cells, similar to the samples shown in Figure 44, there was no significant difference in capacity or resistance of the two cells for EIS run at 0 V bias. The scan was only run to 0.05 Hz so it is possible that this could change for EIS at lower frequency or centered around a different bias. We explored responses to step potential measurements, as shown in Appendix D, showing similar trends. From these measurements so far, we could not conclude that there was any stabilizing difference between the transparent and opaque samples other than a suggestion of asymmetrical voltage difference in each half cell. 4.4.3.6.2 Three electrode measurements Standard three electrode experiments were done by controlling voltages vs. ps-Ag/AgCl to get an even better understanding of electrochemical reactions occurring at each electrode, while also monitoring the counter electrode half-cell. These were different samples from those in the previous section, but prepared in the same way. Three-electrode cyclic voltammograms are shown in Figure 54a. There is a large oxidation peak around 1 V vs. Ag/AgCl after which it appears some electroactivity is lost, and a small reduction peak at -0.7 V attributed to the PEDOT reduction. A small oxidation peak at -0.5 V corresponds to the PEDOT reoxidation. Hydrogen 115 reduction may or may not be present. In Figure 54b-c we observe the contrast L*, half-cell potentials, and current captured during these voltammograms. The transparent sample was observed to have a dramatic rise in current above 0.7 V vs. Ag/AgCl, leading to reduced electroactivity in the second cycle (not shown), so the opaque sample was cycled in a smaller potential window. In both cells we see that as the voltage was varied to reduce the top electrode (first half-cycle), the other electrode again reached a voltage plateau around 0.7 V vs. Ag/AgCl, indicating reaching a redox peak at one electrode. This was more defined as a plateau in these three-electrode measurements, and corresponds to a dramatic increase in effective capacitance, as the electrode will stay at the potential of the reaction until completion. It should be noted that the width of the plateau is wider in Figure 54c because the voltage sweep goes ~100 mV more negative vs. Ag/AgCl, evident in the negative half-cycle in Figure 54a. It is observed that the potential plateau corresponds with the maximum achievable contrast, and this was where the current began to drop. The drop could be because all the available PEDOT at the working electrode (top electrode) has been reduced at this voltage application level. When the top electrode is driven to oxidize, there is a sudden jump in voltage on the opposing side and in current when the top electrode potential goes below -0.7 V vs. Ag/AgCl. The reducing counter electrode must jump to -5 V vs. ps-Ag/AgCl to provide a compensating reaction for what is assumed to be the overoxidation of PEDOT at the top electrode. This indicates possible irreversible chemical damage as the voltage changes steeply to search for a reaction to compensate for the overoxidation of PEDOT- possibly hydrogen Figure 54: a) Cyclic voltammogram three electrode measurements (10 mV/s), b), c) 3 electrode voltage division and optical response across a transparent (b) and an opaque (c) cell, plotting absolute value of voltage vs. ps-Ag/AgCl, top (black) is reducing first 116 reduction, as all the PEDOT has likely already been reduced and is unable to provide charge compensation. This is consistent with the small change in contrast in this voltage region. In terms of the asymmetry noted in the 2-electrode measurements, in these samples the opposite appears to be true- there is greater asymmetry in the opaque sample, but this could be affected by the CV scan window being a different voltage range vs. ps-Ag/AgCl and the overoxidation in the transparent sample. The oxidizing electrode still plateaus at 0.7 V vs. Ag/AgCl in both cases. This appears to contradict the theory that it is TiO2 that causes a more even potential distribution, stabilizing the PEDOT to limit overoxidation. The asymmetry again could be due to different electrode thicknesses and sample variation causing difference in IR drop. Overall, we do not see clear evidence of the action of TiO2 to enhance stability in an electrochemical capacity from these measurements, but we do obtain a look at the half cell potentials of each electrode that may offer clues as to difference in reactions occurring at each electrode. 4.4.3.6.3 Pseudoreference Ag/AgCl measurements summary Using three electrode measurements with pseudoreference Ag/AgCl electrodes we have begun to probe differences in voltage in each electrochromic half-cell and observed asymmetries in voltage distribution. Using this method, we do not reach conclusions on evidence of TiO2 electrochemical participation in the electrochromic cell, which has been shown to have electrochemically stabilizing effects in other electrochromic devices. However, probing the electrode potentials individually as in this approach is important for understanding the stability range, matching electrode capacities in asymmetric cells, and troubleshooting degradation. This is the only example of pseudoreference electrodes used within solid state electrochromic cells that we found. It is also possible that despite our tests in 4.4.3.5 our pseudoreference electrodes were not all fabricated identically and their potentials might vary depending on exposure to the acid electrolyte or on the thickness of the AgCl layer- more tests of controls would have to be performed to confirm this. Certain reactions seem to occur consistently at the same voltages for different samples (for example, the voltage plateaus), suggesting a certain degree of stability. More investigation may be done to show differences in reactions occurring with the addition of the filler that may contribute to increased cycle life, such as 3-electrode results during cycling. 117 With the possibility exposed that material being used up for reduction or oxidation may prevent completion of reactions, there is further evidence that it may be useful to vary thicknesses of the PEDOT layers to provide different amounts of compensating charge. Alternatively, PEDOT cast in a more neutral state between the two stability limits or the addition of a redox mediator such as ferrocene, could provide more counter reaction charge and improve cyclability. If TiO2 does prove useful in stabilizing the cells, this low-cost material could be widely applied in other electrochemical cells. 4.5 Discussion Overall we have demonstrated an intrinsically stretchable electrochromic cell and characterized performance under 30% strain using optical, electrical and electrochemical methods. One caveat is the strain rate must be slow (~10% sample length strain per minute) as the deformation of the PEDOT is largely plastic above ~20% with long relaxation time [6]. In comparing our performance to other flexible PEDOT based cells in literature, we observe our achieved ΔTmax=15% is within the typical range seen in the literature of 15-22%, with the comparable cells all using ITO and thus not stretchable. We have good switch times among average reported results for similar structure devices. Those using ITO can take upwards of 10 seconds to switch, while all our times are <7 s and some ~1 s. Consistency might be higher with improved fabrication to minimize sample to sample differences. Our cyclability in transparent mode is poor, potentially due to degradation of our electrochromic layer, as for electrochromics up to ~1 000 000 cycles have been achieved. Adding redox compensating couples and using wider potential stability window electrolytes could mitigate this. The three-electrode method of testing potentials at each electrode could also help find a good compromise between optical properties and applied voltage range. So far, the opaque samples reported here attain at least 500 cycles with relatively little degradation – with more consistent methods and new experiments needed to see how much longer these samples can be cycled. Even at 500 cycles, samples demonstrate promise for short term use for disposable patches, for example. Our coloration efficiency is (77-194 cm2/C) in the range of other literature values (300-500cm2/C) [7] and could be improved through optimizing layer thicknesses and ensuring the reactions can go to completion with adequate compensating charge. For opaque devices that use 118 a non-transparent white electrolyte and produce contrast from only the top electrode, the best ones in literature have produced contrasts up to ΔE*=33-42 and switch times of <100 ms but these involve ITO. Our device performs well for an unoptimized case with contrast ΔL*max=18 and switch time ~1 s, comparable to some of the other high performing devices in recent literature of similar composition. Improvements could also be made by using some other electrochromic materials, but the stretchability and conductivity would have to be explored in order to know if they could be used in the same capacity. From the fact that the voltage ‘front’ was observed to move across PEDOT in some cases, and for samples with higher resistance electrodes, switch time was longer, it might be expected that the electronic conductivity is a main limiting factor in the switch speed. As the electrolyte resistance high comparable to some liquid electrolytes for electrochromics this is not expected to be a major contributor, but it could include ion conductivity through the electrode to provide compensating charge. It is evident we have created a cell with comparable performance to some recent flexible devices, which is unique in being intrinsically stretchable. This is made possible by being ITO free. With more iteration it might be improved to compare with best PEDOT-based devices in contrast and speed – and add the additional benefit of 30% stretchability, though improvements in cycle stability should be made. Compared to the few other stretchable electrochromic integrated devices – all of which make use of folding or bending of nonstretchable materials - it provides similar speed and contrast, making it a solid candidate for exploration in this field. In our next chapter we explore integrating it into a multi-pixel structure. 119 Chapter 5. Passive matrix display 5.1 Background To develop multifunctional flexible and stretchable displays, after creation of a single pixel, the next step is an addressable multi segment display. This progresses towards displays able to convey complex information such as in smartphones, TVs and e-readers. In this chapter we build upon the single pixel stretchable device developed in Chapter 4 and integrate it into a passive matrix single pixel addressable intrinsically stretchable electrochromic display, also the first of its kind. We first introduce the main three approaches for designing a multi segment display: direct drive, passive matrix and active matrix architectures, requiring different degrees of complexity and control components, and then we choose passive matrix, design a fabrication procedure, describe the testing setup and demonstrate performance and addressability under strain – all working towards an encapsulated wearable device. 5.1.1 Direct drive The simplest architecture for a multi segment display is direct drive, where each pixel has its own positive and negative terminals. This requires the most number of connections and has the slowest update speed, with N2 connections needed for a N×N array [12], and they are generally used for low information devices like digital watches as in Figure 55a. Often multiplexing can be used to control different signals. Direct drive electrochromic devices have been demonstrated in dispenser printed multi-pixel displays and analog number displays [181]. These are applicable to low information wearable and biomedical displays, but development of higher information Figure 55: a) Direct drive 7 segment display, b) passive matrix architecture V/2 protocol, adapted from [12] c) V/3 protocol adapted from [9] 120 displays is limited due to number of connections required. This is one of the easiest concepts to make stretchable because of the simple electrode connections but scaling to large arrays is a challenge. 5.1.2 Passive matrix Passive matrix techniques involve crossing orthogonal row and column electrodes to form a pixel in a grid structure as in Figure 55 [9], [12]. Simultaneously voltage is applied sequentially to entire rows or columns to activate pixels. Other unaddressed pixels along the same line feel partial voltages, so there is a limit in resolution and greyscale capability as cross talk is a major concern. 2N connections are required for a N×N array. A pulse is applied to each row or column for a period, the rate determining the matrix refresh speed. It is highly important for strong contrast for the pixels to have a threshold voltage for turn on of the optoelectronic response curve as in Figure 56, otherwise there will be several levels of grey pixels instead of white or black due to all pixels in the row and column being affected by partial voltages [12]. Ghosting causes major problems of image diffusion and cross talk in voltage driven devices such as electrochromic or electrophoretic displays, the latter of which have so far exhibited no intrinsic voltage threshold [12], [182]. To reduce the cross talk it can be effective to insulate the electrolyte under each pixel from other pixels to minimize image diffusion leakage through ion motion with electric field gradients in the electrolyte, as well as implement a nonlinear threshold in each pixel through asymmetrical cell construction [182]. Figure 56: Passive matrix pixel voltage threshold requirement, reproduced from [12] with permission 121 Passive matrix electrochromic devices have been developed by many groups. Viologen, leuco dye [183]–[186] and PEDOT:PSS based electrochromic passive matrix displays have been demonstrated [9]. In some of these devices methods have been employed to reduce crosstalk, including alterations to materials and architecture, such as incorporating gel electrolytes and isolation of cells [187]. Using dissimilar materials for working and counter electrodes can increase the nonlinear switching behaviour and help create a prominent a threshold voltage. For example, using PEDOT:PSS as one electrode and carbon paste on the other can produce on/off threshold behaviour, as shown in Figure 56 [9]. The update speed increases as the number of pixels increases due to the need to address a single row and column at a time. As switching speed can be seconds to minutes for each pixel, electrochromic passive matrix arrays can be slow, meaning they are often not suitable for moderate to high refresh rate applications. Various voltage application strategies have also been employed to reduce crosstalk, such as employing different voltages to switch pixels depending on the number of pixels activated [184]. Applying partial voltages to unactivated rows and columns such that unactivated pixels experience less voltage difference has been used – for example in the V/3 and V/2 addressing methods [9]. The V/3 protocol is shown in Figure 55c where applying V/3 to unaddressed columns and 2V/3 to unaddressed rows results in each unaddressed pixel feeling a maximum voltage difference of V/3, different from applying 0 voltage in which case column 2 would all experience V. The V/2 protocol is similar but applies V/2 instead of V/3 resulting in unaddressed pixels feeling V/2 voltage difference, placing a larger demand on the threshold voltage. Using this addressing method the group in question was able to produce a messages with low cross talk such as that shown in Figure 57. This type of addressing, with the ability to apply multiple voltage levels to each row and column, places a larger demand on the electronics. An alternative Figure 57: IV behaviour of a) a symmetrical electrochromic PEDOT:PSS pixel without threshold voltage b) a pixel exhibiting threshold voltage using PEDOT:PSS and carbon paste as electrodes, c) printed passive matrix display, reproduced from [9] with permission 122 demonstrated by the same group is to alternate between applying V and a high impedance state connection, characteristic of digital circuitry. Challenges of making a stretchable passive matrix architecture for electrochromics using our already intrinsically stretchable single pixel device are to make the row and column electrodes stretchable, while retaining adequate conductivity and interconnects. An attractive feature of the passive matrix approach is that the number of layers required is similar to that of a single pixel device. Hence, this was the method we chose to use for generating a multi-pixel device. One uncertainty was how well our devices would satisfy the threshold voltage requirement. We were uncertain of the amount of cross talk we would expect - fortunately it is relatively small for single pixel activation, as will be demonstrated. 5.1.3 Active matrix For a passive matrix architecture, each row and column are addressed separately but partial voltages on unaddressed pixels can result in cross talk. An active matrix architecture uses a nonlinear switching element for each pixel to implement a clear threshold turn on voltage to remove this issue. Number of connections is also 2N for a N×N array. Usually a MOSFET thin film transistor is used to address each individual pixel via row and column and they are arranged on a backplane. A typical pixel architecture includes a thin film transistor for each individual pixel and a storage capacitor holds the previous grey level [12]. Several groups have begun development of flexible active matrix electrochromic displays based on thin film transistor backplane arrays. A PEDOT:PSS based device was demonstrated in 2007 by Berggren et al. [10]. Similarly, an 8×8 PEDOT device was shown on polyethylene terephthalate (PET) and another using carbon-nanotube based transistors [188], [189]. However, active matrix devices provide an additional difficulty in progressing to stretchability due to the requirement of the complex transistor backplane architecture to achieve individually activated pixels, as this must also be made stretchable. Much progress has been made on developing stretchable transistors, such as for active matrix OLED displays, but it was not explored in this thesis due to complexity. Without careful fabrication in might be expected that performance with strain would be more variable than a simpler passive matrix architecture due to the concern about each pixel’s threshold. Because of this, we chose to try a passive matrix array. 123 5.2 Fabrication 5.2.1 Matrix Materials used have been previously listed in Chapter 4. Carbon nanofiber (Sigma) was added to the carbon black/SIS/toluene electrodes in a 4:1 wt ratio of carbon black to carbon nanofiber to provide higher conductivity. The steps were employed as in Figure 58. Carbon black row and column electrodes were developed to contact all edges of each pixel to try to ensure adequate connection along the row even if there was mechanical failure in the PEDOT films. Connections were doctor bladed from the carbon black mixture onto a Teflon™ sheet substrate and evaporated for 1 hour, from which patterns were cut out. Polyimide adhesive sheet was cut into a shadow mask for the electrochromic pixels and adhered to 2-inch square SEBS substrates on glass. The spin coating, washing and heating procedure was followed as in Chapter 4 for the single pixel devices, again using 46% BMIM OSU/PEDOT:PSS. The mask was peeled off leaving patterned PEDOT:PSS electrodes, and then then the carbon black electrodes with the TeflonTM sheet backing were aligned with the pixels. They were thermally bonded by clamping and placing in the oven at 100º C for 10 min, after which the TeflonTM backing was peeled off. To prevent contact between row and column carbon black electrodes, a SEBS spacer frame layer, leaving the pixel regions exposed, was added to one side using plasma-assisted thermal bonding. This approach is shown to produce strong bonding in styrene-based Figure 58: Matrix fabrication procedure steps shown 124 thermoplastic elastomers: the spacer side was exposed to 30 W oxygen plasma for 4 min, and the PEDOT layer for only 1 min to avoid plasma etching. The two layers were then clamped together at 70º C for 2 h. In Chapter 4 we discussed SEBS hydrophilicization with plasma to enable strong adhesion of the PEDOT:PSS hydrophilic material. Here we additionally take advantage of this to induce thermoplastic plasma-assisted bonding, which has been shown in other styrene-based thermoplastic elastomers to generate hydrophilic polar groups, and a combination of plasma treatment and thermal bonding have given very high bonding strengths [87], [190]. To pattern the square electrolyte pixels, a similar SEBS spacer frame was pressed firmly in place to act as a mask for PVA evaporation. 200 µL PVA/H3PO4/TiO2 electrolyte solution was spread over each PEDOT:PSS single pixel, and dried according to the procedure in Chapter 4 and then the SEBS mask was removed. The two half-cell matrix layers were air plasma activated for 4 min and then clamped together at 70º C for another 2 h. The exposed carbon black electrode traces were interfaced with digital to analog converters (DACs) on a custom printed circuit board using alligator clips. Figure 59a-b demonstrates the flexibility of the final device. It can be observed that the central pixel is much darker and the other pixels are nonuniform- this is due to nonuniformity in spin coating of the solution and could be mediated by varying the concentration of the components in the solution slightly to reduce early gelation or by moving to blade or spray coating. It can be observed from the images that there were cracks in the pixel areas- this was due to overheating causing slight melting of the SEBS substrate while under pressure, and may have degraded electrical performance, but this could also be mitigated through optimizing the fabrication procedure. No pixels were connected by electrolyte making reduction of cross talk easier. Figure 59: a, b) Demonstrated flexibility of matrix prior to electrical control testing, c) PCB for digital to analog 4×4 device channel control 125 5.2.2 Control matrix PCB A printed circuit board was designed, fabricated and tested in different voltage configurations to implement the passive matrix control, shown in Figure 59c. The voltage drivers were individual 12-bit 4 channel MCP4728 DACs controlled via I2C protocol. One was employed for row and one for column control. Each of these channels could be individually set ‘on’ to a voltage between 0 V to VDD or to high impedance (500 kΩ load) states, controlled using Arduino UNO custom code. This versatile 4×4 channel board could be used to simulate several different voltage application levels like the V/3 or V/2 protocols outlined or for digital control by switching between low impedance voltage application to high impedance states, though more investigation must be done to determine if the high impedance state adequately matched that of digital devices. The time of application of each individual channel could be controlled with ms precision. the control electronics were first validated to ensure voltages were applied as expected by using an LED array and voltmeter, before using the system to test the passive matrix. 5.3 Characterization Electrochemical impedance spectroscopy was performed using a Solartron Analytical 1260A Frequency Response Analyzer controlled by a Solartron 1287 potentiostat galvanostat interface between each row and column to measure impedance of electrical connections. By reprogramming a controlling Arduino UNO to apply different voltages to rows and columns different addressing protocols such as V/2, V/3, or V and high impedance applications, we tried to compare contrast change, speed and degree of cross talk and find the optimal control method for our device. V/3 was chosen as the most successful for recording tests, due to the lower threshold voltage requirement of V/3 and lowest cross talk observed, but sequential row addressing was not yet implemented- only constant voltages to all rows and columns was used. Vstep=1.6 V was used as the switching voltage, with constant 2V/3 and V/3 applied to unaddressed rows and columns according to V/3 protocol to ensure each unaddressed pixel only felt V/3 or -V/3 voltage. Sequential row addressing tuning to allow multiple pixel addressing would be a next step for this to allow diagonal pixel activation and scale-up. For large strain experiments, clamps were attached to a linear actuator driven by an LN-Drive stepper motor driver, controlling a stepper motor and linear stage. The row carbon black contacts were interfaced with copper tape that ran vertically up the side to interface with alligator clips 126 and attached using silver paint, insulated with polyimide tape and coated in LePage 5 minute epoxy to reinforce so that they could be clamped for applying strain, while the top carbon black electrodes to access columns were directly interfaced with alligator clips. VHB 4910 tape was used to improve grip as the matrix was clamped and the stepper motor used to apply strain. An Arduino UNO was used to apply the control signals to the stepper motor and a custom code written to allow control of distance moved with ~200 µm precision and speed measured as 30 µm/s. Strain at this speed was applied in 10% increments with periods of 10 minute rest to allow for relaxation time of the PEDOT film, similar to Chapter 4. At 0%, 10%, 20%, and 30% strain, the V/3 protocol was used to address each pixel to turn on in sequential order. For 20% and 30% strain, the switch step time was increased from 5 s to 10 s per step to accommodate for observed increased time constant to achieve full contrast. Video was recorded according to the process described in Chapter 4 and converted to L* values to determine contrast. Once again, a white Teflon™ sheet background was used as a white reference, as described in Chapter 4. A relative comparison between contrast in 0% and 30% strain and between the pixels was made, as well as observations of relative degree of cross talk. 5.4 Results 5.4.1 Electrochemical impedance spectroscopy From the Nyquist plots in Figure 60 we observe that the resistance across each pixel when measuring impedance between row and column electrodes one by one is around 200-300 Ω. As Figure 60: Nyquist plots for electrical impedance spectroscopy. The real (resistive) and imaginary (capacitive) impedance of each pixel are compared and shown to be similar, denoted by ‘R’ for row and ‘C’ for column number. Inset shows intersection of x axis for all indicating cell resistance ~250 Ω for all pixels. 127 the capacitance is approximately half of that for the cells in Chapter 4, as the imaginary impedance is ~6000 Ω at 0.05 Hz (0.5 mF) and they are half the area, the RC time constant as limited by the sum of the internal resistance and connections and is expected to be ~0.15 s. The contribution of the internal resistance vs. the connections has not yet been measured. The resistance is highest by ~30 Ω for column 3 electrodes, which have the longest row traces. As the matrix is strained along the row axis later, it would be expected that these would experience the largest cell resistance increase due to more cumulative strain along the electrodes. Though the carbon black electrodes are long, they are highly conductive, enabling low resistance along the trace relative to the PEDOT. When measured from the ends of the traces exposed externally, prior to experimentation the resistance along each row or column was ~200 Ω. There was only ~50 Ω variation between the electrodes, surprising given the imprecise nature of the fabrication along with uneven thickness and evaporation rate, though it is unknown how this would change under strain. In an earlier sample in which solely carbon black was used for the stretchable row/column electrodes, the main significant difference was resistance across each pixel was closer to 2 kΩ and the switch time was minimum ~10 s instead of the ~2 s observed in this sample, indicating that resistance of the traces being low is highly important for speed of performance. We would expect that rows or columns with higher resistance would experience slower voltage change due to the time needed for the full voltage difference to be felt as current flows- initially most of the voltage drop may be across the electrodes and not the electrochromic layers, meaning the electrochemical state will be closer to the initial condition (e.g. 0 V for a cell that is off) and the contrast change will take longer to proceed as the ions are not drawn as strongly to compensate the charge. The resulting RC time constant can be reduced by further increasing contact conductivity, reducing pixel capacity, and lowering the internal resistance of each cell. 5.4.2 Matrix addressing control Our original cell required only 1.6 V to switch and this was used as Vstep. From electrochemical impedance spectroscopy the resistance across each pixel was similar to that in single pixel experiments due to the high conductivity carbon black/carbon fiber/SIS electrodes addressing rows and columns, and since time for switch is dependent on R we expected the switching times to be similar. 128 We tried 3 different addressing protocols: digital, V/2, and V/3, but without full development of the proper row sequential addressing and duty cycle electronic control necessary to provide full passive matrix functionality, including complete unique pixel addressability- only constant voltage application. Digital and V/2 protocol addressing methods were tried with constant voltage application rather than sequential row addressing, but it was found they could not produce single pixel addressability. To simulate digital circuit operation we tried alternating off or on application to each row and column sequentially, while keeping others in a high impedance state, but this resulted in entire rows or columns changing colour due to crosstalk as the unaddressed pixels along the active row or column still felt the applied voltage despite the high impedance state, causing charge flow through the cell. The high impedance was only 500 kΩ. The RC time constant associated with this impedance would be expected to be >100 s such that cross talk signal might appear with long application. We tried the V/2 protocol, but again this resulted in entire rows and columns changing at once, due to the unaddressed items feeling V/2 voltage and no real threshold voltage of our cell as discussed in Chapter 4. The V/3 protocol in the end allowed us to simply apply constant partial voltages to all rows and columns and activate single pixels. This approach was what we used for our strain tests. This is not the standard operation procedure for passive matrix architecture but is a step in this direction. It does not allow for unique multiple pixel addressing along the diagonal. Further development of duty cycle by optimizing update frequency and voltage levels with sequential row addressing would be the next step. According to previous work [9] we should be able to accomplish the necessary partial voltage levels for complete unique addressability through sequential row addressing changing VRMS values for each pixel, though cross talk may be higher due to our less established threshold voltage. To test with our method, we activated pixels 1 by 1 in sequence as shown in Figure 61 to observe pixel contrast and degree of cross talk, by setting the addressed column and row to 1.6 V difference with our DAC control board, and the other rows and columns to partial voltages 2V/3 and V/3 so that unaddressed pixels only felt V/3 voltage difference. The V/3 protocol gave the ability to activate single pixels as shown in Figure 61, where each cell can be observed changing as it is addressed. In testing, each cell was activated for 5 s (0% strain) or 10 s (30% strain) before deactivating and moving on to the next. 129 We observe calculated contrast in Figure 62 to be in the range for single pixels shown in Chapter 4 for opaque cells. Using process described in 4.3.1.1 we calculated approximate L*a*b* values from the sRGB values of the video. Each of the lines represents the luminance L* of a different pixel, numbered 1-9 as shown. Similar to Chapter 4 we consider that there may be uncertainty in the value of L* due to the inconsistent lighting on the sample and white Teflon™ background, for example column 3 being offset to a brighter value than other pixels. Generally, Figure 61: Activation of individual pixels using constant partial voltage addressing V/3 protocol under a) 0% strain, and b) 30% strain. We observe that single pixels activated in order 1-9 demonstrate single pixel addressability both in their undeformed and deformed state, demonstrating our final intrinsically stretchable multi-pixel electrochromic display. 130 the contrast change observed of ΔL*=20-40 is similar to values reported for single opaque pixels in Chapter 4, indicating consistent contrast with the single pixels in the matrix form. It is noticed that the contrast is different for different pixels- the central pixel experiences a 30% larger change (ΔL*~10) as well as being darker to begin with. This can be attributed to PEDOT film nonuniformity from the spin coating application. Small amounts of cross talk can be observed in Figure 62c and d, by looking at the change in contrast of the other pixels while the addressed pixel is changing to the largest extent. For example, in Figure 62c, while pixel 8 is activated, the light green trace, we also see changes in the contrasts of pixel 2 and 5 along the same column and pixel 7 and 9, along the same row. Central pixel 5 is observed to have the largest contribution to cross talk when partially activated. For the single electrochromic pixel, the contrast did not show a clear threshold voltage, as was discovered in the three electrode cyclic voltammetric measurements in Chapter 4 Figure 53, though contrast was observed to increase faster after approximately 1 V, so a degree of cross talk is expected in passive matrix addressing. Hence our observed slight contrast change for unaddressed pixels along the same row and column as the activated pixel feels V/3 voltage is to Figure 62: Luminosity L* and Change in Luminosity under 0% and 30% strain for each pixel 1-9 131 be expected. From the contrast L* plots we observe that pixels that are unaddressed experience mostly ΔL*<5 compared to the addressed pixel ΔL*=20-40 indicating presence of cross talk but to a low extent. Given that L* is supposed to linear in the human perception for contrast, we would expect that the crosstalk-induced contrast seen by the eye would be <1/4 of the contrast experience by the addressed pixels. The central pixel showed the largest contribution to crosstalk when unaddressed due to increased PEDOT content, demonstrating the importance of pixel uniformity. Other considerations that could contribute to different crosstalk might include nonuniform thickness of the carbon electrodes, nonuniform thickness of the electrolyte layer, and different contact resistance that would change the ratio of voltage division along each column and row, causing higher resistance channels to experience smaller voltage at the active region at the beginning. This could cause varying partial contrast changes while current is flowing if the charge time is not long enough for the current to subside. The time constant was surprisingly fast and observed visually for each pixel to be ~3 s. In depth evaluation of rate limiting factors was not performed at this stage. From these measurements we concluded with constant partial voltage addressing we could achieve low crosstalk single pixel activation addressing of our as-fabricated matrix. Next we investigate if this performance could be maintained under strain. 5.4.3 Performance under 30% strain In Figure 61b and Figure 62b, d, under 30% strain case we observe that when the pixels are addressed one by one in a similar way, we get similar addressability of single pixels, indicating the successful single pixel activation operation of the passive matrix display under strain. It was observed that the colour change speed was slower, so the step size was increased to 10 s per switch. It was also observed that the strain was nonuniform across the sample, with pixels in column 3 experiencing ~50% strain and the pixels in column 1 experiencing <30% strain. This is due to nonuniform thickness of the sample- it was thinner on the column 3 side due to nonuniform thickness of the cast SEBS substrates. The fact that the higher strained pixels still experience contrast change and are addressable indicates potentially that the PEDOT cells could be strained >30% strain and still operate. However, we stopped at 30% strain due to some carbon electrode breakage between column 2 and column 3 observed- the point of failure was in the carbon electrodes not the PEDOT pixels. 132 Visually, the contrast observed in Figure 61 is lower for the highly strained pixels that are sequentially addressed, but still allows for addressability with low cross talk. For the calculated contrast shown in Figure 62 we observe the baselines are moved to higher luminosity for all pixels. The highest contrast is again at pixel 5 (the centre) and is ~35 – almost as high as for the unstretched case. This demonstrates what is possible. The relatively high contrast of this pixel is likely due to increased content of PEDOT, as discussed above. The stretching is expected to have some effect on contrast. In strain along the horizontal axis pictured we would expect resistance to mainly increase in the rows, creating asymmetrical voltage distribution and higher resistance for current flow in one direction. For the switching, in column 1, the post-strain contrast is decreased by only ΔL*~3-5, while for column 3 it is decreased up to ΔL*~10. This lower contrast could be both due to the fact that the strain is higher, resulting in less PEDOT per area changing colour, and also due to the fact that the carbon row electrodes are experiencing the most cumulative strain and resistance increase at column 3 resulting in the full 1.6 V voltage drop not being felt at these pixels in the early charging stages, so that it will take longer to go to completion. Even without breakage, the increased resistance along the row due to the strained carbon electrodes could result in part of the voltage drop being across the row instead of the pixel, resulting in lower contrast change in the given step time before current flow stops when the voltage of the entire electrode goes to 0 or 1.6 V. Under strain there was low crosstalk. However, because the contrast for each pixel was reduced, the crosstalk contrast was a larger proportion of the ΔL*, up to >1/3 of the addressed pixel contrast, such as in pixel 6, causing the contrast between different pixels to be not as apparent. In the central pixel, the contrast to crosstalk ratio was greater than ~10, again showing what is achievable with some improvement in the design and fabrication. Time constant was visually observed to increase, and so a longer switch time was allocated (10 s) per pixel in order to observe more full contrast change. In some cases luminosity was still changing at the end of 10 s (e.g. pixel 5, as seen in Figure 62c-d). In most cases the time was still observed to be <5 s and similar between pixels. It was longest for the column 3 electrodes- at the end of the highly strained and breaking carbon electrodes. Since the time constant is proportional to RC, we would expect that because these pixels are at the end of the electrodes and they have the most strained PEDOT pixels (with the PEDOT layer resistance expected to increase with stretch). they would experience the largest increase in R and time constant. As we did not 133 measure change in the carbon electrode resistance with strain we are unsure of the increase in row resistance. The PEDOT increase in resistance was measured as discussed in 4.4.3.1 and shown in Figure 44 to increase approximately 16-40 Ω/cm with 1 cm width at 30% strain, such that maximum change in resistance along the row due to the 3 PEDOT pixels might be expected to be 50-120 Ω and would contribute a change in RC time constant of up to 0.06 s for the farthest pixel along the row- any other increase might be due to the carbon electrode. An earlier prototype exhibited similar operation after 6-month storage in air, demonstrating a reasonably long ambient lifetime of the materials in the device. 5.5 Discussion Overall, the device well demonstrates a prototype of an intrinsically stretchable PEDOT-based electrochromic passive matrix display. Going beyond the demonstrated first intrinsically stretchable electrochromic pixel described in Chapter 4, the array demonstrates applicability in a multi-pixel display. The contact between the carbon and PEDOT layers was a soft material to soft material contact and therefore very well ensured remaining contact under strain- a successful approach to the soft-to-hard interconnect problem, as the soft carbon electrodes then interfaced externally with hard copper electrodes in a non-strained region. Nevertheless, for device construction, we should explore an alteration of the carbon electrode to allow strain beyond 30% without breakage- here this was one of the limiting factors for device strain rather than strain of the PEDOT layer. Investigation and optimization of resistance with strain would give a better understanding of what voltage to apply to rapidly achieve 1.6 V across the cell while under strain without having to wait long charging times for the current to subside. We could also envision applying resistance compensation – an extra voltage that is applied initially in order to overcome IR drop and speed charging, such as reported by Madden et al. for fast actuating polypyrrole actuators of similar layered structure [191]. To improve contrast and uniformity of appearance, the PEDOT layer top electrode and counter electrode thicknesses should be optimized for high contrast as well as strain. To get around the nonuniformity from spin-coating, a more uniform coating method could be implemented like spray coating or blade coating. 134 For crosstalk, uniformity of the pixels and electrodes is very important, as demonstrated by the increased cross talk observed for the higher PEDOT content pixel in the center. This changes the voltage distribution along each row and column and can cause variations in the expected V/3 maximum voltage to be felt in unaddressed pixels while there is current prior to full charging. Also, for pixels with thicker PEDOT layers, the switch is expected to occur at different voltages due to slightly different threshold voltage [123]. Creating asymmetric electrodes with different thicknesses of PEDOT or incorporating other materials such as using carbon as counter electrode as in [9] may also improve the threshold behaviour. Consistency of fabrication to ensure uniformly manufactured pixels and electrodes must be achieved in order to determine the degree of crosstalk for a more ideally uniform matrix. Time constant we expect to be increased under strain mainly due to increase in resistance and RC. Therefore, to reduce time constant we would want to find an electrode material, potentially carbon or silver nanomaterial based, that has very low increase in resistance with strain. We still must investigate exactly what was the increase in this case to be able to gauge requirements. We could also explore applying varying voltages under strain. Though the V/3 protocol is useful here for addressing individual pixels, in the digital age it is not very practical for large format displays to have 4 voltage levels – V, V/3, 2V/3, 0 – for matrix device operation. It does not seamlessly fit into the conventional large-scale fabrication of the semiconductor industry using mainly digital on/off switches and requires more electronic components. As mentioned, we also did not yet explore complete pixel unique addressability for multi-pixel activation, even with the partial V/3 protocol and instead applied voltage to all rows and columns at once. First, sequential row activation with duty cycle to achieve pixel unique addressability should be tested, with either the V/3, V/2 or digital addressing protocols to see if low crosstalk multiple pixel activation can be achieved by the device. It is possible our threshold voltage may not be adequate to perform this in which case the device architecture must be redesigned. If this is successful, digital solutions with V alternating with high impedance or 0 V application should be further explored as this minimizes the electronics required, facilitating miniaturization and may allow better transition to matrix consumer display devices. Active matrix devices could also be considered but require more complex circuitry [10] that would be 135 difficult to make highly intrinsically stretchable with a TFT backplane, with an example recent advance shown in [192] of a 100% stretchable organic transistor array. The passive matrix operation demonstrated here is currently adequate for low-information and slow update displays, given ~5 s switch time and not for video rate fast update. In the conclusion we provide approximate specifications we expect we could achieve with the current prototype and what example applications might be. Overall, we demonstrate a passive matrix single pixel addressable intrinsically stretchable electrochromic device prototype using simple solution processing and thermal bonding techniques, potentially suitable for slow update displays for wearables, biomedical monitoring patches, and robotic skin. 136 Chapter 6. Conclusion In this thesis we have developed an intrinsically stretchable electrochromic device. It is the first to have integrated interconnects and show encapsulated performance under 30% strain. We also used it to develop the first intrinsically stretchable encapsulated 3×3 matrix single pixel addressable display and demonstrated operation under 30% strain. Overall, our device demonstrates contrast, speed and low crosstalk comparable to some high performing PEDOT-based flexible devices in the literature while also demonstrating intrinsic stretchability and performance on the order of or exceeding other stretchable electrochromic materials or devices. In Chapter 2 we used two recently developed approaches to produce intrinsically stretchable PEDOT-containing films, by either combining PEDOT with insulating elastomer, or PEDOT:PSS with dopants. We fabricated them and characterized their relative conductivity, electrochromic performance, stretchability, and electrochemical properties and chose the doped PEDOT:PSS film. This film enabled the construction of a device with intrinsic stretchability, and usage of the film as an electrode, an electrochromic layer and an ion storage layer, resulting in simplified device architecture. In Chapter 3 we fabricated several stretchable ionic conductors to look for a compatible partner with our selected electrochromic film. They were compared for electrochemical compatibility for electrochromic switching with high speed and contrast, as well as mechanical durability in forming a bond with the PEDOT. A PVA/H3PO4 electrolyte was selected for high electrochemical compatibility and mechanical durability from amongst the candidates. In Chapter 4 we developed electrodes and a fabrication procedure for the full display device in single pixel form for both transparent and reflective devices. To measure its performance under strain we had to develop and characterize accuracy of a custom optical, mechanical, electrochemical setup. We then demonstrated device operation under strain showing minimal performance degradation, characterized its optical, electrical, and electrochemical properties and demonstrated moderate cyclability. We also newly demonstrated the usage of a pseudoreference electrode in a solid state electrochromic cell to measure half cell potentials as a useful method to investigate degradation mechanisms. In Chapter 5 we took the single pixel and developed it into an addressable stretchable passive matrix. The array demonstrated single pixel addressable high contrast operation with our chosen 137 voltage addressing scheme under both 0% and 30% strain with low switch time <7 s and with only some performance degradation under strain. In the field of stretchable displays, we have demonstrated the first intrinsically stretchable electrochromic display device with integrated electrode contacts in both single pixel and passive matrix display form. The electrochromic active area is all polymer, and the contacts are integrated, not needing metal-based additional conductive electrodes. It switches quickly among conventional electrochromic devices due to the conductive PEDOT, also quickly under strain with minimal degradation in performance. We have also demonstrated operation of an encapsulated multi-pixel display in combination with carbon electrodes, a major step towards application in a wearable device. We showed that the active materials in the device can be stretchable enough intrinsically to not largely limit the performance of the device, and not require pre-strain or extendable geometry to achieve a stretchable device. This is remarkable given conventional electrical performance degradation upon strain of many materials. Previously this intrinsically stretchable polymer film was demonstrated for transparent stretchable electrode purposes and now we extended this to show its applicability in a display device, also adding its electrochromic capabilities. The performance is rapid, exhibiting ability to change contrast in ~5 s without an additional electrode layer, and contrast comparable to the average contrasts of some other flexible electrochromic devices, though not yet the best reported devices based on PEDOT. We also were able to come up with a method to quantify the performance while under strain by designing a new experimental setup, and using a linear actuator to apply strain. Though we demonstrate this advance, our device has limitations due to material choices and lack of optimization. Contrast is not as high as the best PEDOT devices of similar flexible but not stretchable construction but could be improved by altering thicknesses of the layers. The speed is acceptable only for devices that update with a refresh rate of seconds or minutes, but not as fast as some sub-100 ms devices based on nanostructured electrochromic films that might approach suitability for video rate displays. These items could be improved by optimizing the layer thicknesses, nanostructure and processing conditions of the current device. The electrolyte chosen limits the electrochemical stability window and the cycling stability. So far 500 electrochemical switching cycles have been recorded with only modest degradation. It is possible that a different choice of electrolyte with a larger stability window, and possibly the 138 addition of redox mediators, the cycling stability could be greatly improved - towards commercializable levels. Additional conductive material could be added to the electrodes to ensure electrical properties stay high quality even if PEDOT degrades with cycling. Biocompatibility of the electrolyte is also a concern that could be dealt with by choosing the material carefully. The solid-state nature of the device is a great strength, in that devices were shown to work even after 4 months storage in air with similar to original performance speed and contrast. Evaporation of the electrolyte is not a concern, and air stability is evident. Mechanically, the PEDOT film used here was previously reported to have a 70 s relaxation time and largely plastic deformation after 20% strain [6]. This limits the mechanical durability of the device and the speed at which it can be strained. It is possible as studies continue to progress in doping and plasticization of conducting polymers, that composites with improved elasticity and shorter relaxation times that retain high conductivity might be developed, but until then we are limited to the material properties of our main active material or applying geometrical buckling to impart additional stretchability. Coupling with a nanomaterial composite stretchable transparent electrode could help to mitigate this by allowing conductive paths to remain even if there are microcracks in the intrinsically stretchable film but this removes the all-polymer low-cost aspect. There is often also mechanical fracture at the contacts, despite the usage of the soft carbon black elastomer composite. This bond could be further improved for a better graded soft to hard interconnect. It is also likely that the contacts may delaminate from the PEDOT over time, causing device failure especially with parasitic hydrogen gas generation. The contact quality could be improved by optimizing the bonding procedure and material and ensuring isolation from the electrolyte. For the development of a matrix device, though we demonstrated high success in low crosstalk addressable switching of single pixels, there were issues with fabrication as well as contrast inconsistencies across the matrix. We also did not implement a fully unique addressability control method allowing multiple pixel addressing, which would be the next step towards a conventional passive matrix device. Improvement of consistency of pixels, interconnects and electrodes must be achieved to obtain more uniform operation over the array and could be done by moving to larger scale and more automated fabrication methods. The small 139 crosstalk may be reduced by optimizing layer thicknesses and material choice to produce a more defined threshold voltage and avoid partial voltages partially activating unaddressed pixels. In terms of applications, we have demonstrated a prototype of a device with wide applications to many wearable, biomedical monitoring, and robotic skin areas. In its current form, the device could be immediately be suitable for low power, short term, disposable displays. This could be used in biomedical monitoring, such as integrated into sensing multifunctional platforms as indicator patches for blood sugar or heart rate as a cheap, conformable, low power display that is comfortable to wear on the skin [1]. For many applications, flexibility is enough for most requirements and stretchability is only occasionally required if deformed beyond normal parameters to prevent functional failure. End material cost is estimated to be <$0.02/cm2 for the active area if the fabrication procedure can be optimized to reduce waste material, indicating high affordability for small patch areas. Wearable devices of disposable nature could also be used as small low power devices, with an estimated use of ~1 mW/cm2 based on switching contrast every 10 s and <10 mC charge transfer, compared to low power OLED displays requiring ~10 mW/cm2 in regular usage conditions [193]. If all materials are nontoxic it could also be highly useful in packaging indicators such as temperature history indicating shipping labels for thermally sensitive materials. Poor cyclability with the current version limits long term usage, but operation after 6 months storage in ambient air indicates long shelf life. For wearables the elastomeric nature limits breathability compared to fabric, but skin-like camouflage coverings could be used on people or objects to change visual appearance with low power usage, especially if it is not required to change the appearance over long time periods, in which case there will be very low power usage. Other fabric based electrochromics such as those with spandex might be more suited for clothing [50]. For robotic skin this provides the capability for visual indication of any quantity across a robotic skin surface with a soft electronic property. It could potentially deform with the robot motion providing more bio-inspired and seamless integration. Other applications could include smart packaging or other flexible consumer electronics- any display applications favouring low power, simple information conveyance, conformability and durability. Future directions to take this device would first involve implementing some of the above discussed improvements to make the device more biocompatible, stable and cyclable including optimizing the electrolyte, layer thicknesses and contacts. Further exploration of material variation and processing as well as control methods to probe applicability as a passive matrix, 140 direct drive or active matrix could be employed. In parallel, tests can begin to be done in incorporating additional colours of alternate thiophene-derivative electrochromic materials to make multi-colour displays. The device could also begin to be implemented in exploration with flexible and stretchable sensors and biomedical monitoring devices to make multifunctional stretchable touch sensing displays that can conform with motion and be integrated on the skin of a human body or onto robotic manipulators. Overall, we have demonstrated this first prototype of this novel device and now look towards integration into stretchable electronics, extending the capability of electronic devices into a new realm of mechanical compliance and changing the technology landscape to become softer, more like us. 141 References [1] J. A. Rogers, T. Someya, and Y. Huang, “Materials and Mechanics for Stretchable Electronics,” Science, vol. 327, no. 5973. pp. 1603–1607, 2010, doi: 10.1126/science.1182383. [2] S. Bauer, S. Bauer-Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger, and R. Schwödiauer, “25th anniversary article: A soft future: From robots and sensor skin to energy harvesters,” Adv. Mater., vol. 26, no. 1, pp. 149–162, 2014, doi: 10.1002/adma.201303349. [3] T. Sekitani et al., “Stretchable active-matrix organic light-emitting diode display using printable elastic conductors,” Nat. Mater., vol. 8, no. 6, pp. 494–499, 2009, doi: 10.1038/nmat2459. [4] T. Cheng, Y. Zhang, W. Y. Lai, and W. Huang, “Stretchable thin-film electrodes for flexible electronics with high deformability and stretchability,” Adv. Mater., vol. 27, no. 22, pp. 3349–3376, 2015, doi: 10.1002/adma.201405864. [5] H. Kai, W. Suda, Y. Ogawa, K. Nagamine, and M. Nishizawa, “Intrinsically Stretchable Electrochromic Display by a Composite Film of Poly(3,4-ethylenedioxythiophene) and Polyurethane,” ACS Appl. Mater. Interfaces, vol. 9, no. 23, pp. 19513–19518, 2017, doi: 10.1021/acsami.7b03124. [6] Y. Wang et al., “A highly stretchable, transparent, and conductive polymer,” Sci. Adv., vol. 3, no. 3, pp. 1–11, 2017, doi: 10.1126/sciadv.1602076. [7] R. Singh, J. Tharion, S. Murugan, and A. Kumar, “ITO-Free Solution-Processed Flexible Electrochromic Devices Based on PEDOT:PSS as Transparent Conducting Electrode,” ACS Appl. Mater. Interfaces, vol. 9, no. 23, pp. 19427–19435, 2017, doi: 10.1021/acsami.6b09476. [8] M. Mallouki, P.-H. Aubert, L. Beouch, F. Vidal, and C. Chevrot, “Symmetrical electrochromic device from poly(3,4-(2,2-dimethylpropylenedioxy)thiophene)-based semi-interpenetrating polymer network,” Synth. Met., vol. 162, no. 21–22, pp. 1903–1911, 2012, doi: 10.1016/j.synthmet.2012.08.006. [9] P. Andersson, J. Kawahara, and M. Berggren, “Printed passive matrix addressed electrochromic displays,” Org. Electron., vol. 14, no. 12, pp. 3371–3378, 2013, doi: 10.1016/j.orgel.2013.10.008. [10] B. P. Andersson, R. Forchheimer, P. Tehrani, and M. Berggren, “Printable All-Organic Electrochromic Active-Matrix Displays,” Adv. Funct. Mater., pp. 3074–3082, 2007, doi: 10.1002/adfm.200601241. [11] X. Cao et al., “Fully Screen-Printed, Large-Area, and Flexible Active-Matrix 142 Electrochromic Displays Using Carbon Nanotube Thin-Film Transistors,” ACS Nano, vol. 10, no. 11, pp. 9816–9822, 2016, doi: 10.1021/acsnano.6b05368. [12] J. Chen, W. Cranton, and M. Fihn, Handbook of visual display technology, vol. 1–4. Berlin Heidelberg: Springer, 2012. [13] J. Wang, C. Yan, K. J. Chee, and P. S. Lee, “Highly stretchable and self-deformable alternating current electroluminescent devices,” Adv. Mater., vol. 27, no. 18, pp. 2876–2882, 2015, doi: 10.1002/adma.201405486. [14] J. X. Wang, C. Y. Yan, G. F. Cai, M. Q. Cui, A. L. S. Eh, and P. S. Lee, “Extremely Stretchable Electroluminescent Devices with Ionic Conductors,” Adv. Mater., vol. 28, no. 22, pp. 4490–4496, 2016, doi: 10.1002/adma.201504187. [15] C. Larson et al., “Highly stretchable electroluminescent skin for optical signaling and tactile sensing,” Science, vol. 351, no. 6277. pp. 1071–1074, 2016, doi: 10.1126/science.aac5082. [16] S. Li, B. N. Peele, C. M. Larson, H. Zhao, and R. F. Shepherd, “A Stretchable Multicolor Display and Touch Interface Using Photopatterning and Transfer Printing,” Adv. Mater., pp. 1–6, 2016, doi: 10.1002/adma.201603408. [17] R. H. Kim et al., “Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics,” Nat. Mater., vol. 9, no. 11, pp. 929–937, 2010, doi: 10.1038/nmat2879. [18] A. Sugimoto, H. Ochi, S. Fujimura, A. Yoshida, T. Miyadera, and M. Tsuchida, “Flexible OLED displays using plastic substrates,” IEEE J. Sel. Top. Quantum Electron., vol. 10, no. 1, pp. 107–114, 2004, doi: 10.1109/JSTQE.2004.824112. [19] T. Yokota et al., “Ultraflexible organic photonic skin,” Sci. Adv., vol. 2, no. 4, pp. 1–8, 2016, doi: 10.1126/sciadv.1501856. [20] M. S. White et al., “Ultrathin, highly flexible and stretchable PLEDs,” Nat. Photonics, vol. 7, no. 10, pp. 811–816, 2013, doi: 10.1038/nphoton.2013.188. [21] R. P. Xu, Y. Q. Li, and J. X. Tang, “Recent advances in flexible organic light-emitting diodes,” J. Mater. Chem. C, vol. 4, no. 39, pp. 9116–9142, 2016, doi: 10.1039/c6tc03230c. [22] Z. Luo and J. Yurek, “Quantum dots: The technology platform for all future displays,” vol. 1094010, no. March, p. 36, 2019, doi: 10.1117/12.2508088. [23] T. H. Kim et al., “Full-colour quantum dot displays fabricated by transfer printing,” Nat. Photonics, vol. 5, no. 3, pp. 176–182, 2011, doi: 10.1038/nphoton.2011.12. [24] Y. Shirasaki, G. J. Supran, M. G. Bawendi, and V. Bulović, “Emergence of colloidal quantum-dot light-emitting technologies,” Nat. Photonics, vol. 7, no. 1, pp. 13–23, 2013, doi: 10.1038/nphoton.2012.328. 143 [25] X. Yang et al., “Highly flexible, electrically driven, top-emitting, quantum dot light-emitting stickers,” ACS Nano, vol. 8, no. 8, pp. 8224–8231, 2014, doi: 10.1021/nn502588k. [26] M. K. Choi et al., “Wearable red–green–blue quantum dot light-emitting diode array using high-resolution intaglio transfer printing,” Nat. Commun., vol. 6, no. May, p. 7149, 2015, doi: 10.1038/ncomms8149. [27] Q. Pei, Y. Yang, G. Yu, C. Zhang, and A. J. Heeger, “Polymer light-emitting electrochemical cells: In situ formation of a light-emitting p-n junction,” J. Am. Chem. Soc., vol. 118, no. 16, pp. 3922–3929, 1996, doi: 10.1021/ja953695q. [28] Z. Yu, M. Sun, and Q. Pei, “Electrochemical formation of stable p-i-n junction in conjugated polymer thin films,” J. Phys. Chem. B, vol. 113, no. 25, pp. 8481–8486, 2009, doi: 10.1021/jp902343e. [29] Z. Yu, M. Wang, G. Lei, J. Liu, L. Li, and Q. Pei, “Stabilizing the dynamic p-i-n junction in polymer light-emitting electrochemical cells,” J. Phys. Chem. Lett., vol. 2, no. 5, pp. 367–372, 2011, doi: 10.1021/jz200071b. [30] T. Sakanoue et al., “High Current Injection into Dynamic p–n Homojunction in Polymer Light-Emitting Electrochemical Cells,” Adv. Mater., vol. 29, no. 18, pp. 2–6, 2017, doi: 10.1002/adma.201606392. [31] J. Liang, L. Li, X. Niu, Z. Yu, and Q. Pei, “Elastomeric polymer light-emitting devices and displays,” Nat. Photonics, vol. 7, no. 10, pp. 817–824, 2013, doi: 10.1038/nphoton.2013.242. [32] H. L. Filiatrault, G. C. Porteous, R. S. Carmichael, G. J. E. Davidson, and T. B. Carmichael, “Stretchable light-emitting electrochemical cells using an elastomeric emissive material,” Adv. Mater., vol. 24, no. 20, pp. 2673–2678, 2012, doi: 10.1002/adma.201200448. [33] Z. K. Tan et al., “Bright light-emitting diodes based on organometal halide perovskite,” Nat. Nanotechnol., vol. 9, no. 9, pp. 687–692, 2014, doi: 10.1038/nnano.2014.149. [34] Y. F. Li et al., “Stretchable Organometal-Halide-Perovskite Quantum-Dot Light-Emitting Diodes,” Adv. Mater., vol. 1807516, pp. 1–6, 2019, doi: 10.1002/adma.201807516. [35] J. A. Rogers et al., “Paper-like electronic displays: Large-area rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks,” Proc. Natl. Acad. Sci. U. S. A., vol. 98, no. 9, pp. 4835–4840, 2001, doi: 10.1073/pnas.091. [36] T. Kitamura, T. Nakamura, N. Kobayashi, T. Sawada, T. Abe, and S. Yoshioka, “Stretchable and flexible electrophoretic image display,” 21st Int. Disp. Work. 2014, IDW 2014, vol. 2, pp. 1156–1157, 2014. [37] M. A. Mossman, V. H. Kwong, and L. A. Whitehead, “A novel reflective image display 144 using total internal reflection,” Displays, vol. 25, no. 5, pp. 215–221, 2004, doi: 10.1016/j.displa.2004.09.020. [38] P. R. Somani and S. Radhakrishnan, “Electrochromic Materials and Devices,” Mater. Chem. Phys., vol. 77, pp. 1–638, 2002, doi: 10.1002/9783527679850. [39] J. a. Castellano, Handbook of Display Technology. San Jose, California: Academic Press, Inc, 1992. [40] A. L. S. Eh, A. W. M. Tan, X. Cheng, S. Magdassi, and P. S. Lee, “Recent Advances in Flexible Electrochromic Devices: Prerequisites, Challenges, and Prospects,” Energy Technol., vol. 6, no. 1, pp. 33–45, 2018, doi: 10.1002/ente.201700705. [41] J. Jensen, M. Hösel, A. L. Dyer, and F. C. Krebs, “Development and manufacture of polymer-based electrochromic devices,” Adv. Funct. Mater., vol. 25, no. 14, pp. 2073–2090, 2015, doi: 10.1002/adfm.201403765. [42] G. Cai et al., “Highly Stable Transparent Conductive Silver Grid/PEDOT:PSS Electrodes for Integrated Bifunctional Flexible Electrochromic Supercapacitors,” Adv. Energy Mater., vol. 6, no. 4, pp. 1–8, 2016, doi: 10.1002/aenm.201501882. [43] S. V. Vasilyeva, P. M. Beaujuge, S. Wang, J. E. Babiarz, V. W. Ballarotto, and J. R. Reynolds, “Material strategies for black-to-transmissive window-type polymer electrochromic devices,” ACS Appl. Mater. Interfaces, vol. 3, no. 4, pp. 1022–1032, 2011, doi: 10.1021/am101148s. [44] Q. Pei, G. Zuccarello, M. Ahlskog, and O. Inganäs, “Electrochromic and highly stable poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue,” Polymer (Guildf)., vol. 35, no. 7, pp. 1347–1351, 1994, doi: 10.1016/0032-3861(94)90332-8. [45] R. H. Bulloch, J. A. Kerszulis, A. L. Dyer, and J. R. Reynolds, “Mapping the broad CMY subtractive primary color gamut using a dual-Active electrochromic device,” ACS Appl. Mater. Interfaces, vol. 6, no. 9, pp. 6623–6630, 2014, doi: 10.1021/am500290d. [46] R. H. Bulloch, J. A. Kerszulis, A. L. Dyer, and J. R. Reynolds, “An electrochromic painter’s palette: Color mixing via solution co-processing,” ACS Appl. Mater. Interfaces, vol. 7, no. 3, pp. 1406–1412, 2015, doi: 10.1021/am507514z. [47] J. A. Kerszulis, K. E. Johnson, M. Kuepfert, D. Khoshabo, A. L. Dyer, and J. R. Reynolds, “Tuning the painter’s palette: Subtle steric effects on spectra and colour in conjugated electrochromic polymers,” J. Mater. Chem. C, vol. 3, no. 13, pp. 3211–3218, 2015, doi: 10.1039/c4tc02685c. [48] C. Yan et al., “Stretchable and Wearable Electrochromic Devices,” ACS Nano, no. 1, pp. 316–322, 2014. [49] H.-H. Chou et al., “A chameleon-inspired stretchable electronic skin with interactive 145 colour changing controlled by tactile sensing,” Nat. Commun., vol. 6, p. 8011, 2015, doi: 10.1038/ncomms9011. [50] M. A. Invernale, Y. Ding, and G. A. Sotzing, “All-Organic electrochromic spandex,” ACS Appl. Mater. Interfaces, vol. 2, no. 1, pp. 296–300, 2010, doi: 10.1021/am900767p. [51] A. Elschner; S. Kirchmeyer; W. L¨ovenich; U. Merker; K. Reuter, PEDOT: Principles and application of an Intrinsically Conducting Polymer. Boca Raton, FL: CRC Press, Taylor and Francis Group, LLC, 2011. [52] Y. Li, Lecture Notes in Chemistry 91: Organic Optoelectronic Materials. Switzerland: Springer International Publishing, 2015. [53] T. A. Skotheim, E. Elsenbaumer, and J. R. Reynolds, Handbook of Conducting Polymers, 2nd ed., vol. 15, no. 1. New York: Marcel Dekker, 1999. [54] J. C. Gustafsson, B. Liedberg, and O. Inganfis, “In situ spectroscopic investigations of electrochromism and ion transport in a poly ( 3 , 4-ethylenedioxythiophene ) electrode in a solid state electrochemical cell,” Solid State Ionics, vol. 69, pp. 145–152, 1994. [55] A. Kumar, D. M. Welsh, M. C. Morvant, F. Piroux, K. A. Abboud, and J. R. Reynolds, “Conducting Poly(3,4-alkylenedioxythiophene) Derivatives as Fast Electrochromics with High-Contrast Ratios,” Chem. Mater., vol. 10, no. 3, pp. 896–902, 1998, doi: 10.1021/cm9706614. [56] H. W. Heuer, R. Wehrmann, and S. Kirchmeyer, “Electrochromic window based on conducting poly(3,4-ethylenedioxythiophene)- poly(styrene sulfonate),” Adv. Funct. Mater., vol. 12, no. 2, pp. 89–94, 2002, doi: 10.1002/1616-3028(20020201)12:2<89::AID-ADFM89>3.0.CO;2-1. [57] R. Gangopadhyay, B. Das, and M. R. Molla, “How does PEDOT combine with PSS? Insights from structural studies,” RSC Adv., vol. 4, no. 83, pp. 43912–43920, 2014, doi: 10.1039/c4ra08666j. [58] R. J. Murphy et al., “Scattering Studies on Poly(3,4-ethylenedioxythiophene)-Polystyrenesulfonate in the Presence of Ionic Liquids,” Macromolecules, vol. 48, no. 24, pp. 8989–8997, 2015, doi: 10.1021/acs.macromol.5b02320. [59] A. J. Bard and L. R. Faulkner, Electrochemical Methods Fundamentals and Applications. New York, NY: John Wiley & Sons, 2001. [60] T. A. Skotheim and J. R. Reynolds, Conjugated polymers: Processing and applications, vol. 2. Boca Raton, FL: CRC Press, 2007. [61] P. Kumar, Z. Yi, S. Zhang, A. Sekar, F. Soavi, and F. Cicoira, “Effect of channel thickness, electrolyte ions, and dissolved oxygen on the performance of organic electrochemical transistors,” Appl. Phys. Lett., vol. 107, no. 5, pp. 6–11, 2015, doi: 10.1063/1.4927595. 146 [62] M. Dietrich, J. Heinze, G. Heywang, and F. Jonas, “Electrochemical and spectroscopic characterization of polyalkylenedioxythiophenes,” J. Electroanal. Chem., vol. 369, no. 1–2, pp. 87–92, 1994, doi: 10.1016/0022-0728(94)87085-3. [63] M. F. Zainal and Y. Mohd, “Characterization of PEDOT Films for Electrochromic Applications,” Polym. - Plast. Technol. Eng., vol. 54, no. 3, pp. 276–281, 2015, doi: 10.1080/03602559.2014.977422. [64] S. Sindhu, K. Narasimha Rao, S. Ahuja, A. Kumar, and E. S. R. Gopal, “Spectral and optical performance of electrochromic poly(3,4-ethylenedioxythiophene) (PEDOT) deposited on transparent conducting oxide coated glass and polymer substrates,” Mater. Sci. Eng. B Solid-State Mater. Adv. Technol., vol. 132, no. 1–2, pp. 39–42, 2006, doi: 10.1016/j.mseb.2006.02.030. [65] A. V. Volkov et al., “Understanding the Capacitance of PEDOT:PSS,” Adv. Funct. Mater., vol. 27, no. 28, pp. 1–10, 2017, doi: 10.1002/adfm.201700329. [66] R. V. Apraksin, A. I. Volkov, S. N. Eliseeva, and V. V. Kondratiev, “Influence of addition of lithium salt solution into PEDOT:PSS dispersion on the electrochemical and spectroscopic properties of film electrodes,” J. Solid State Electrochem., vol. 21, no. 12, pp. 3487–3494, 2017, doi: 10.1007/s10008-017-3694-0. [67] C. Badre, L. Marquant, A. M. Alsayed, and L. A. Hough, “Highly conductive poly(3,4-ethylenedioxythiophene):Poly (styrenesulfonate) films using 1-ethyl-3-methylimidazolium tetracyanoborate ionic liquid,” Adv. Funct. Mater., vol. 22, no. 13, pp. 2723–2727, 2012, doi: 10.1002/adfm.201200225. [68] Q. Wei, M. Mukaida, Y. Naitoh, and T. Ishida, “Morphological change and mobility enhancement in PEDOT:PSS by adding co-solvents,” Adv. Mater., vol. 25, no. 20, pp. 2831–2836, 2013, doi: 10.1002/adma.201205158. [69] S. Mukherjee et al., “Solution-processed poly(3,4-ethylenedioxythiophene) thin films as transparent conductors: Effect of p-toluenesulfonic acid in dimethyl sulfoxide,” ACS Appl. Mater. Interfaces, vol. 6, no. 20, pp. 17792–17803, 2014, doi: 10.1021/am504150n. [70] J. Rivnay et al., “Structural control of mixed ionic and electronic transport in conducting polymers,” Nat. Commun., vol. 7, pp. 1–9, 2016, doi: 10.1038/ncomms11287. [71] B. J. Worfolk et al., “Ultrahigh electrical conductivity in solution-sheared polymeric transparent films,” Proc. Natl. Acad. Sci. U. S. A., vol. 112, no. 46, pp. 14138–14143, 2015, doi: 10.1073/pnas.1509958112. [72] M. Y. Teo et al., “Highly stretchable and highly conductive PEDOT:PSS/Ionic liquid composite transparent electrodes for solution-processed stretchable electronics,” ACS Appl. Mater. Interfaces, vol. 9, no. 1, pp. 819–826, 2017, doi: 10.1021/acsami.6b11988. [73] S. Savagatrup et al., “Plasticization of PEDOT:PSS by common additives for mechanically robust organic solar cells and wear able sensors,” Adv. Funct. Mater., vol. 147 25, no. 3, pp. 427–436, 2015, doi: 10.1002/adfm.201401758. [74] M. Vosgueritchian, D. J. Lipomi, and Z. Bao, “Highly conductive and transparent PEDOT:PSS films with a fluorosurfactant for stretchable and flexible transparent electrodes,” Adv. Funct. Mater., vol. 22, no. 2, pp. 421–428, 2012, doi: 10.1002/adfm.201101775. [75] J. Onorato, V. Pakhnyuk, and C. K. Luscombe, “Structure and design of polymers for durable, stretchable organic electronics,” Polym. J., vol. 49, no. 1, pp. 41–60, 2017, doi: 10.1038/pj.2016.76. [76] L. Zhao, J. Qiu, X. Cao, W. Dong, J. You, and Y. Li, “Ionic liquid induced supramolecular self-assembly of poly(3,4- ethylenedioxythiophene)/poly(4-styrenesulfonate) thin films with enhanced conductivity and tunable nanoporosity,” Macromol. Res., vol. 21, no. 4, pp. 456–461, 2013, doi: 10.1007/s13233-013-1087-5. [77] S. Kee et al., “Controlling Molecular Ordering in Aqueous Conducting Polymers Using Ionic Liquids,” Adv. Mater., vol. 28, no. 39, pp. 8625–8631, 2016, doi: 10.1002/adma.201505473. [78] M. Döbbelin, R. Marcilla, C. Tollan, J. A. Pomposo, J. R. Sarasua, and D. Mecerreyes, “A new approach to hydrophobic and water-resistant poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) films using ionic liquids,” J. Mater. Chem., vol. 18, no. 44, pp. 5354–5358, 2008, doi: 10.1039/b808723g. [79] M. A. Leaf and M. Muthukumar, “Electrostatic Effect on the Solution Structure and Dynamics of PEDOT:PSS,” Macromolecules, vol. 49, no. 11, pp. 4286–4294, 2016, doi: 10.1021/acs.macromol.6b00740. [80] D. J. Lipomi, J. A. Lee, M. Vosgueritchian, B. C. K. Tee, J. A. Bolander, and Z. Bao, “Electronic properties of transparent conductive films of PEDOT:PSS on stretchable substrates,” Chem. Mater., vol. 24, no. 2, pp. 373–382, 2012, doi: 10.1021/cm203216m. [81] G. J. N. Wang, A. Gasperini, and Z. Bao, “Stretchable Polymer Semiconductors for Plastic Electronics,” Adv. Electron. Mater., vol. 4, no. 2, pp. 1–21, 2018, doi: 10.1002/aelm.201700429. [82] W. Yin, C. an Tao, F. Wang, J. Huang, T. Qu, and J. Wang, “Tuning optical properties of MOF-based thin films by changing the ligands of MOFs,” Sci. China Mater., vol. 61, no. 3, pp. 391–400, 2018, doi: 10.1007/s40843-017-9143-5. [83] J. S. Noh, “Highly conductive and stretchable poly(dimethylsiloxane):poly(3,4- ethylenedioxythiophene):poly(styrene sulfonic acid) blends for organic interconnects,” RSC Adv., vol. 4, no. 4, pp. 1857–1863, 2014, doi: 10.1039/c3ra46087h. [84] T. S. Hansen, K. West, O. Hassager, and N. B. Larsen, “Highly stretchable and conductive polymer material made from poly(3,4-ethylenedioxythiophene) and polyurethane elastomers,” Adv. Funct. Mater., vol. 17, no. 16, pp. 3069–3073, 2007, doi: 148 10.1002/adfm.200601243. [85] P. Li, K. Sun, and J. Ouyang, “Stretchable and Conductive Polymer Films Prepared by Solution Blending,” ACS Appl. Mater. Interfaces, vol. 7, no. 33, pp. 18415–18423, 2015, doi: 10.1021/acsami.5b04492. [86] C. A. Lamont, O. Winther-Jensen, and B. Winther-Jensen, “A conductive stretchable PEDOT-elastomer hybrid with versatile processing and properties,” J. Mater. Chem. B, vol. 3, no. 43, pp. 8445–8448, 2015, doi: 10.1039/c5tb02009c. [87] S. Yu, S. P. Ng, Z. Wang, C. L. Tham, and Y. C. Soh, “Thermal bonding of thermoplastic elastomer film to PMMA for microfluidic applications,” Surf. Coatings Technol., vol. 320, pp. 437–440, 2017, doi: 10.1016/j.surfcoat.2016.11.102. [88] C. W. Tsao and D. L. DeVoe, “Bonding of thermoplastic polymer microfluidics,” Microfluid. Nanofluidics, vol. 6, no. 1, pp. 1–16, 2009, doi: 10.1007/s10404-008-0361-x. [89] “MFP-3D Manual Version 04_08.” Asylum Research Atomic Force Microscopes, 2008. [90] M. Döbbelin et al., “Influence of ionic liquids on the electrical conductivity and morphology of PEDOT:PSS films,” Chem. Mater., vol. 19, no. 9, pp. 2147–2149, 2007, doi: 10.1021/cm070398z. [91] S. Bindu and M. Suresh, “Ohmic Contact Formation to PEDOT-PSS Films Using Graphite-Clay,” Br. J. Appl. Sci. Technol., vol. 6, no. 4, pp. 342–349, 2015, doi: 10.9734/bjast/2015/13337. [92] F. M. Smits, “Measurement of Sheet Resistivities with the Four-Point Probe,” Bell Syst. Tech. J., vol. 37, pp. 711–718, 1958. [93] Y. Hui, C. Bian, J. Wang, J. Tong, and S. Xia, “Comparison of two types of overoxidized PEDOT films and their application in sensor fabrication,” Sensors, vol. 17, no. 3, pp. 2–12, 2017, doi: 10.3390/s17030628. [94] M. A. Kamensky, S. N. Eliseeva, G. Láng, M. Ujvári, and V. V. Kondratiev, “Electrochemical Properties of Overoxidized Poly-3,4-Ethylenedioxythiophene,” Russ. J. Electrochem., vol. 54, no. 11, pp. 893–901, 2018, doi: 10.1134/S1023193518130219. [95] P. Y. Pennarun, S. Papaefthimiou, P. Yianoulis, and P. Jannasch, “Electrochromic devices operating with electrolytes based on boronate ester compounds and various alkali metal salts,” Sol. Energy Mater. Sol. Cells, vol. 91, no. 4, pp. 330–341, 2007, doi: 10.1016/j.solmat.2006.09.009. [96] V. K. Thakur, G. Ding, J. Ma, P. S. Lee, and X. Lu, “Hybrid materials and polymer electrolytes for electrochromic device applications,” Adv. Mater., vol. 24, no. 30, pp. 4071–4096, 2012, doi: 10.1002/adma.201200213. [97] Y. Huang et al., “An Intrinsically Stretchable and Compressible Supercapacitor 149 Containing a Polyacrylamide Hydrogel Electrolyte,” Angew. Chemie - Int. Ed., vol. 56, no. 31, pp. 9141–9145, 2017, doi: 10.1002/anie.201705212. [98] K. Fujii et al., “High-performance ion gel with tetra-PEG network,” Soft Matter, vol. 8, no. 6, pp. 1756–1759, 2012, doi: 10.1039/c2sm07119c. [99] A. Maziz et al., “Demonstrating kHz frequency actuation for conducting polymer microactuators,” Adv. Funct. Mater., vol. 24, no. 30, pp. 4851–4859, 2014, doi: 10.1002/adfm.201400373. [100] P. Tamilarasan and S. Ramaprabhu, “Stretchable supercapacitors based on highly stretchable ionic liquid incorporated polymer electrolyte,” Mater. Chem. Phys., vol. 148, no. 1–2, pp. 48–56, 2014, doi: 10.1016/j.matchemphys.2014.07.010. [101] J. X. Wang, C. Y. Yan, G. F. Cai, M. Q. Cui, A. L. S. Eh, and P. S. Lee, “Extremely Stretchable Electroluminescent Devices with Ionic Conductors,” Adv. Mater., vol. 28, no. 22, pp. 4490–4496, 2016, doi: 10.1002/adma.201504187. [102] C. S. Kim and S. M. Oh, “Spectroscopic and electrochemical studies of PMMA-based gel polymer electrolytes modified with interpenetrating networks,” J. Power Sources, vol. 109, no. 1, pp. 98–104, 2002, doi: 10.1016/S0378-7753(02)00055-1. [103] J. Guan, Y. Li, and J. Li, “Stretchable Ionic-Liquid-Based Gel Polymer Electrolytes for Lithium-Ion Batteries,” Ind. Eng. Chem. Res., vol. 56, no. 44, pp. 12456–12463, 2017, doi: 10.1021/acs.iecr.7b03387. [104] X. Wang, C. Yang, J. Jin, X. Li, Q. Cheng, and G. Wang, “High-performance stretchable supercapacitors based on intrinsically stretchable acrylate rubber/MWCNTs@conductive polymer composite electrodes,” J. Mater. Chem. A, vol. 6, no. 10, pp. 4432–4442, 2018, doi: 10.1039/c7ta11173h. [105] S. Choudhury, T. Saha, K. Naskar, M. Stamm, G. Heinrich, and A. Das, “A highly stretchable gel-polymer electrolyte for lithium-sulfur batteries,” Polymer, vol. 112. Elsevier Ltd, pp. 447–456, 2017, doi: 10.1016/j.polymer.2017.02.021. [106] C. Zhao, C. Wang, Z. Yue, K. Shu, and G. G. Wallace, “Intrinsically stretchable supercapacitors composed of polypyrrole electrodes and highly stretchable gel electrolyte,” ACS Appl. Mater. Interfaces, vol. 5, no. 18, pp. 9008–9014, 2013, doi: 10.1021/am402130j. [107] K. P. Singh, R. P. Singh, and P. N. Gupta, “Polymer based solid state electrochromic display device using PVA complex electrolytes,” Solid State Ionics, vol. 78, no. 3–4, pp. 223–229, 1995, doi: 10.1016/0167-2738(95)00100-K. [108] Y. Huang et al., “A self-healable and highly stretchable supercapacitor based on a dual crosslinked polyelectrolyte,” Nat. Commun., vol. 6, 2015, doi: 10.1038/ncomms10310. [109] Y. Yokoyama, T. Fukutsuka, K. Miyazaki, and T. Abe, “Origin of the electrochemical 150 stability of aqueous concentrated electrolyte solutions,” J. Electrochem. Soc., vol. 165, no. 14, pp. A3299–A3303, 2018, doi: 10.1149/2.0491814jes. [110] G. Vaivars, A. Azens, and C. G. Granqvist, “Proton conducting polymer composites for electrochromic devices,” Solid State Ionics, vol. 119, no. 1, pp. 269–273, 1999, doi: 10.1016/S0167-2738(98)00513-X. [111] B. Liu, Y. Y. Zhang, X. H. Zhang, B. Y. Du, and Z. Q. Fan, “Fixation of carbon dioxide concurrently or in tandem with free radical polymerization for highly transparent polyacrylates with specific UV absorption,” Polym. Chem., vol. 7, no. 22, pp. 3731–3739, 2016, doi: 10.1039/c6py00525j. [112] K. Rohtlaid, “Fabrication, characterization and integration of microactuators and microsensors based on electronically conducting polymers,” Science and Engineering, University of Cergy-Pontoise. [113] H. Seok Jang, C. Justin Raj, W. G. Lee, B. Chul Kim, and K. Hyun Yu, “Enhanced supercapacitive performances of functionalized activated carbon in novel gel polymer electrolytes with ionic liquid redox-mediated poly(vinyl alcohol)/phosphoric acid,” RSC Adv., vol. 6, no. 79, pp. 75376–75383, 2016, doi: 10.1039/c6ra15070e. [114] Y. Taeeun, Y. C. Chang, M. Junyoung, M. O. Seung, and G. K. Young, “Synthesis and properties of acyclic ammonium-based ionic liquids with allyl substituents as electrolytes,” Molecules, vol. 14, no. 5, pp. 1840–1851, 2009, doi: 10.3390/molecules14051840. [115] D. Eric Shen, A. M. Österholm, and J. R. Reynolds, “Out of sight but not out of mind: The role of counter electrodes in polymer-based solid-state electrochromic devices,” J. Mater. Chem. C, vol. 3, no. 37, pp. 9715–9725, 2015, doi: 10.1039/c5tc01964h. [116] E. Roy, M. Geissler, J. C. Galas, and T. Veres, “Prototyping of microfluidic systems using a commercial thermoplastic elastomer,” Microfluid. Nanofluidics, vol. 11, no. 3, pp. 235–244, 2011, doi: 10.1007/s10404-011-0789-2. [117] P. Verge et al., “Symmetrical Electrochromic and Electroemissive Devices from Semi-Interpenetrating Polymer Networks,” Adv. Sci. Technol. StafaZuerich Switz., vol. 55, no. Smart Optics, pp. 18–23, 2008, doi: 10.4028/www.scientific.net/AST.55.18. [118] D. Mecerreyes et al., “A simplified all-polymer flexible electrochromic device,” vol. 49, pp. 3555–3559, 2004, doi: 10.1016/j.electacta.2004.03.032. [119] S. Gong, L. W. Yap, B. Zhu, and W. Cheng, “Multiscale Soft–Hard Interface Design for Flexible Hybrid Electronics,” Adv. Mater., vol. 1902278, pp. 1–28, 2019, doi: 10.1002/adma.201902278. [120] V. S. Chakravarthi, Y. J. M. Shirur, and P. Rekha, “Development of Technique for Making Ohmic Contacts to PEDOT:PSS Films,” pp. 61–70, 2013, doi: 10.1007/978-81-322-1524-0. 151 [121] M. Kateb, V. Ahmadi, and M. Mohseni, “Fast switching and high contrast electrochromic device based on PEDOT nanotube grown on ZnO nanowires,” Sol. Energy Mater. Sol. Cells, vol. 112, pp. 57–64, 2013, doi: 10.1016/j.solmat.2013.01.021. [122] A. S. Shaplov et al., “A first truly all-solid state organic electrochromic device based on polymeric ionic liquids,” Chem. Commun., vol. 50, no. 24, pp. 3191–3193, 2014, doi: 10.1039/C3CC49876J. [123] J. Kawahara, P. Andersson, I. Engquist, and M. Berggren, “Improving the color switch contrast in PEDOT : PSS-based electrochromic displays,” Org. Electron., vol. 13, no. 3, pp. 469–474, 2012, doi: 10.1016/j.orgel.2011.12.007. [124] X. H. Xia et al., “Electrochemistry Communications Multicolor and fast electrochromism of nanoporous NiO / poly ( 3 , 4-ethylenedioxythiophene ) composite thin film,” Electrochem. commun., vol. 11, no. 3, pp. 702–705, 2009, doi: 10.1016/j.elecom.2009.01.017. [125] V. Hern, “Ultrafast and High-Contrast Electrochromism on Bendable Transparent Carbon Nanotube Electrodes,” pp. 1367–1371, 2010, doi: 10.1021/jz100333s. [126] S. L. Cho and S. B. Lee, “Fast Electrochemistry of Conductive Polymer Nanotubes : Synthesis, Mechanism , and Application,” Acc. Chem. Res., vol. 41, no. 6, pp. 699–707, 2008. [127] A. Takahashi, “One million cyclable blue/colourless electrochromic device using K2Zn3[Fe(CN)6]2 nanoparticles synthesized with a micromixer,” RSC Adv., vol. 9, pp. 41083–41087, 2019, doi: 10.1039/c9ra09496b. [128] C. G. Granqvist, Handbook of Inorganic Electrochromic Materials, 2nd ed. Amsterdam: Elsevier, 2002. [129] A. Kariman and A. T. Marshall, “ Improving the Stability of DSA Electrodes by the Addition of TiO 2 Nanoparticles ,” J. Electrochem. Soc., vol. 166, no. 8, pp. E248–E251, 2019, doi: 10.1149/2.0761908jes. [130] C. J. Tang, J. M. Ye, Y. T. Yang, and J. L. He, “Large-area flexible monolithic ITO/WO3/Nb2O5/NiVOχ/ITO electrochromic devices prepared by using magnetron sputter deposition,” Optical Materials, vol. 55. Elsevier B.V., pp. 83–89, 2016, doi: 10.1016/j.optmat.2016.03.021. [131] M. Mizuhashi, J. Nagai, and T. Kamimori, “Electrochromic device design towards large-scale applications,” Proc. SPIE 10304, Large-Area Chromogenics Mater. Devices Transm. Control, vol. 10304, no. March 1990, p. 103040V, 1990, doi: 10.1117/12.2283634. [132] S. Chalupczok, P. Kurzweil, and H. Hartmann, “Impact of Various Acids and Bases on the Voltammetric Response of Platinum Group Metal Oxides,” Int. J. Electrochem., vol. 2018, no. 2, pp. 1–6, 2018, doi: 10.1155/2018/1697956. 152 [133] R. T. Wen, C. G. Granqvist, and G. A. Niklasson, “Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO 3 thin films,” Nat. Mater., vol. 14, no. 10, pp. 996–1001, 2015, doi: 10.1038/nmat4368. [134] J. Zhao, G. Wang, Q. Zhang, W. Rui, C. Qu, and P. Gao, “An underlying intercalation ion for fast-switching and stable electrochromism,” J. Mater. Sci. Mater. Electron., vol. 30, no. 13, pp. 12753–12756, 2019, doi: 10.1007/s10854-019-01640-2. [135] J. Remmele, D. E. Shen, T. Mustonen, and N. Fruehauf, “High Performance and Long-Term Stability in Ambiently Fabricated Segmented Solid-State Polymer Electrochromic Displays,” ACS Appl. Mater. Interfaces, vol. 7, no. 22, pp. 12001–12008, 2015, doi: 10.1021/acsami.5b02090. [136] D. Chatzikyriakou et al., “Mesoporous amorphous tungsten oxide electrochromic films: A Raman analysis of their good switching behavior,” Electrochim. Acta, vol. 137, pp. 75–82, 2014, doi: 10.1016/j.electacta.2014.05.139. [137] I. Winter, C. Reese, J. Hormes, G. Heywang, and F. Jonas, “The thermal ageing of poly(3,4-ethylenedioxythiophene). An investigation by X-ray absorption and X-ray photoelectron spectroscopy,” Chem. Phys., vol. 194, no. 1, pp. 207–213, 1995, doi: 10.1016/0301-0104(95)00026-K. [138] M. Vázquez, J. Bobacka, A. Ivaska, and A. Lewenstam, “Influence of oxygen and carbon dioxide on the electrochemical stability of poly(3,4-ethylenedioxythiophene) used as ion-to-electron transducer in all-solid-state ion-selective electrodes,” Sensors Actuators, B Chem., vol. 82, no. 1, pp. 7–13, 2002, doi: 10.1016/S0925-4005(01)00983-2. [139] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D. D. C. Bradley, and J. R. Durrant, “Degradation of organic solar cells due to air exposure,” Sol. Energy Mater. Sol. Cells, vol. 90, no. 20, pp. 3520–3530, 2006, doi: 10.1016/j.solmat.2006.06.041. [140] N. Massonnet, A. Carella, A. de Geyer, J. Faure-Vincent, and J.-P. Simonato, “Metallic Behaviour in Acid Doped Highly Conductive Polymers,” R. Soc. Chem., vol. 00, 2012, doi: 10.1039/x0xx00000x. [141] C.-C. Chen, C.-Y. Chiang, T.-Y. Wu, and I.-W. Sun, “Improved Electrochromic Properties of Poly(3,4-ethylenedioxythiophene) in 1-butyl-3-methylimidazolium Dicyanamide,” ECS Electrochem. Lett., vol. 2, no. 10, pp. H43–H45, 2013, doi: 10.1149/2.009310eel. [142] S. M. Kim et al., “Influence of PEDOT:PSS crystallinity and composition on electrochemical transistor performance and long-term stability,” Nat. Commun., vol. 9, no. 1, 2018, doi: 10.1038/s41467-018-06084-6. [143] M. Forsyth, D. R. MacFarlane, A. Best, J. Adebahr, P. Jacobsson, and A. J. Hill, “The effect of nano-particle TiO2 fillers on structure and transport in polymer electrolytes,” Solid State Ionics, vol. 147, no. 3–4, pp. 203–211, 2002, doi: 10.1016/S0167-2738(02)00017-6. 153 [144] W. Liu et al., “Ionic Conductivity Enhancement of Polymer Electrolytes with Ceramic Nanowire Fillers,” Nano Lett., vol. 15, no. 4, pp. 2740–2745, 2015, doi: 10.1021/acs.nanolett.5b00600. [145] W. Wang and P. Alexandridis, “Composite polymer electrolytes: Nanoparticles affect structure and properties,” Polymers, vol. 8, no. 11. 2016, doi: 10.3390/polym8110387. [146] S. Hashimoto and H. Matsuoka, “Prolonged Lifetime of Electrochromism,” Surf. Interface Anal., vol. 19, no. 1992, pp. 464–468, 1992. [147] K. R. Reyes-Gil, Z. D. Stephens, V. Stavila, and D. B. Robinson, “Composite WO3/TiO2 nanostructures for high electrochromic activity,” ACS Appl. Mater. Interfaces, vol. 7, no. 4, pp. 2202–2213, 2015, doi: 10.1021/am5050696. [148] Y. Liu et al., “Enhanced Dispersion of TiO2 Nanoparticles in a TiO2/PEDOT:PSS Hybrid Nanocomposite via Plasma-Liquid Interactions,” Sci. Rep., vol. 5, no. March, pp. 1–11, 2015, doi: 10.1038/srep15765. [149] W. Maiaugree, S. Pimanpang, M. Towannang, S. Saekow, W. Jarernboon, and V. Amornkitbamrung, “Optimization of TiO 2 nanoparticle mixed PEDOT-PSS counter electrodes for high efficiency dye sensitized solar cell,” J. Non. Cryst. Solids, vol. 358, no. 17, pp. 2489–2495, 2012, doi: 10.1016/j.jnoncrysol.2011.12.104. [150] R. S. Ajimsha et al., “Band offset at TiO2/MDMO PPV and TiO2/PEDOT PSS interfaces studied using photoelectron spectroscopy,” RSC Adv., vol. 5, no. 118, pp. 97891–97897, 2015, doi: 10.1039/c5ra21227h. [151] K. Trzciński, M. Szkoda, A. P. Nowak, M. Łapiński, and A. Lisowska-Oleksiak, “Widening of the electroactivity potential range by composite formation - capacitive properties of TiO2/BiVO4/PEDOT:PSS electrodes in contact with an aqueous electrolyte,” Beilstein J. Nanotechnol., vol. 10, pp. 483–493, 2019, doi: 10.3762/BJNANO.10.49. [152] D. Amorello, V. Gambino, V. Romano, and R. Zingales, “The formal redox potential of the Ti(IV, III) couple at 25°C in 1 M HCl 2 M NaCl medium,” Ann. Chim., vol. 97, no. 8, pp. 713–722, 2007, doi: 10.1002/adic.200790055. [153] K. Trzciński and A. Lisowska-Oleksiak, “Electrochemical characterization of a composite comprising PEDOT/PSS and N doped TiO2 performed in aqueous and non-aqueous electrolytes,” Synth. Met., vol. 209, pp. 399–404, 2015, doi: 10.1016/j.synthmet.2015.08.026. [154] K. K. Kasem and S. Jones, “Platinum as a reference electrode in electrochemical measurements,” Platin. Met. Rev., vol. 52, no. 2, pp. 100–106, 2008, doi: 10.1595/147106708X297855. [155] M. A. Pellitero et al., “Quantitative self-powered electrochromic biosensors,” Chem. Sci., vol. 8, no. 3, pp. 1995–2002, 2017, doi: 10.1039/c6sc04469g. 154 [156] C. Xu, L. Liu, S. E. Legenski, D. Ning, and M. Taya, “Switchable window based on electrochromic polymers,” J. Mater. Res., vol. 19, no. 7, pp. 2072–2080, 2004, doi: 10.1557/JMR.2004.0259. [157] W. Li et al., “Thieno[3,2-b] thiophene-based conjugated copolymers for solution-processable neutral black electrochromism,” Polym. Chem., vol. 9, no. 47, pp. 5608–5616, 2018, doi: 10.1039/c8py01208c. [158] M. Waleed Shinwari, D. Zhitomirsky, I. A. Deen, P. R. Selvaganapathy, M. Jamal Deen, and D. Landheer, “Microfabricated reference electrodes and their biosensing applications,” Sensors, vol. 10, no. 3, pp. 1679–1715, 2010, doi: 10.3390/s100301679. [159] N. Rohaizad, C. C. Mayorga-Martinez, F. Novotný, R. D. Webster, and M. Pumera, “3D-printed Ag/AgCl pseudo-reference electrodes,” Electrochem. commun., vol. 103, no. April, pp. 104–108, 2019, doi: 10.1016/j.elecom.2019.05.010. [160] E. T. S. G. Da Silva, S. Miserere, L. T. Kubota, and A. Merkoçi, “Simple on-plastic/paper inkjet-printed solid-state Ag/AgCl pseudoreference electrode,” Anal. Chem., vol. 86, no. 21, pp. 10531–10534, 2014, doi: 10.1021/ac503029q. [161] Y. K. Lee, T. H. Yoon, B. S. Lim, C. W. Kim, and J. M. Powers, “Effects of colour measuring mode and light source on the colour of shade guides,” J. Oral Rehabil., vol. 29, no. 11, pp. 1099–1107, 2002, doi: 10.1046/j.1365-2842.2002.00961.x. [162] K. Nassau, Color for Science, Art and Technology. Amsterdam: Elsevier, 1998. [163] M. Gargano, N. Ludwig, and D. Pandini, “Use of optical fibre in spectrometry and colorimetry with remote probes,” J. Int. Colour Assoc., vol. 8, pp. 36–43, 2012. [164] F. G. Rosillo and M. C. Alonso-García, “Evaluation of color changes in PV modules using reflectance measurements,” Sol. Energy, vol. 177, pp. 531–537, 2019, doi: 10.1016/j.solener.2018.11.039. [165] M. D. Fairchild, Color Appearance Models, 2nd. Ed. Chichester: John Wiley and Sons, Ltd, 2013. [166] M. Anderson, R. Motta, and S. C. M. Stokes, “A Standard Default Color Space for the Internet - sRGB,” Color Imaging Conf., vol. 8, pp. 238–245, 1996. [167] O. Noboru and A. R. Robertson, Colorimetry Fundamentals and Applications. Chichester: John Wiley & Sons Ltd, 2005. [168] I. C. Monge-Romero and M. F. Suárez-Herrera, “Electrocatalysis of the hydroquinone/benzoquinone redox couple at platinum electrodes covered by a thin film of poly(3,4-ethylenedioxythiophene),” Synth. Met., vol. 175, pp. 36–41, 2013, doi: 10.1016/j.synthmet.2013.04.027. [169] L. K. Mestha and S. A. Dianat, Control of Color Imaging Systems: Analysis and Design. 155 Boca Raton, FL: CRC Press Taylor and Francis Group, 2009. [170] “Documentation: Image Processing Toolbox,” MathWorks, 2019. [Online]. Available: https://www.mathworks.com/help/images/. [Accessed: 20-Jan-2020]. [171] S. Westland and C. Ripamonti, Computational Colour Science Using MATLAB. Chichester: John Wiley & Sons Ltd, 2004. [172] F. M. Mirabella, Modern Techniques in Applied Molecular Spectroscopy. New York, NY: John Wiley and Sons, 1998. [173] S. D. Noble and T. G. Crowe, “Sample holder and methodology for measuring the reflectance and transmittance of narrow-leaf samples,” Appl. Opt., vol. 46, no. 22, pp. 4968–4976, 2007, doi: 10.1364/AO.46.004968. [174] A. S. Jerry Workman, Jr., Applied Spectroscopy: A Compact Reference for Practitioners. Chestnut Hill, MA: Academic Press, 1998. [175] E. Udd, “Optical fiber sensors,” Fiber Integr. Opt., vol. 11, no. 4, pp. 319–336, 1992, doi: 10.1080/01468039208204198. [176] A. Hovi, P. Forsström, M. Mõttus, and M. Rautiainen, “Evaluation of accuracy and practical applicability of methods for measuring leaf reflectance and transmittance spectra,” Remote Sens., vol. 10, no. 1, 2018, doi: 10.3390/rs10010025. [177] M. Potůčková et al., “Comparison of reflectance measurements acquired with a contact probe and an integration sphere: Implications for the spectral properties of vegetation at a leaf level,” Sensors, vol. 16, no. 11, 2016, doi: 10.3390/s16111801. [178] W. J. Mccarthy and S. Lowry, “Technical Note 51695: Comparing the Performance of a Fiber Optic Probe to an Integrating Sphere.” Thermo Scientific, 2008. [179] N. T. Nguyen, “Ultrathin conducting polymer transducers: fabrication, characterization, and modeling,” University of British Columbia, 2018. [180] S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, and H. A. Gasteiger, “A Gold Micro-Reference Electrode for Impedance and Potential Measurements in Lithium Ion Batteries,” J. Electrochem. Soc., vol. 163, no. 10, pp. A2265–A2272, 2016, doi: 10.1149/2.0581610jes. [181] Y. Wei, X. Wang, R. Torah, and J. Tudor, “Dispenser printing of electrochromic display on textiles for creative applications,” Electron. Lett., vol. 53, no. 12, pp. 779–781, 2017, doi: 10.1049/el.2017.0119. [182] A. E. Aliev and H. W. Shin, “Image diffusion and cross-talk in passive matrix electrochromic displays,” Displays, vol. 23, no. 5, pp. 239–247, 2002, doi: 10.1016/S0141-9382(02)00052-5. 156 [183] M. O. M. Edwards, G. Thuraisingham, A. Hagfeldt, T. Gruszecki, and H. Pettersson, “P-92 : Electrochromic Passive-Matrix Displays,” SID 03 Dig., pp. 570–571, 2003. [184] M. O. M. Edwards, “Passive-matrix addressing of viologen- Ti O2 displays,” Appl. Phys. Lett., vol. 86, no. 7, pp. 1–3, 2005, doi: 10.1063/1.1865334. [185] W. Weng et al., “A high-speed passive-matrix electrochromic display using a mesoporous TiO2 electrode with vertical porosity,” Angew. Chemie - Int. Ed., vol. 49, no. 23, pp. 3956–3959, 2010, doi: 10.1002/anie.200907008. [186] N. Kobayashi, S. Miura, M. Nishimura, and H. Urano, “Organic electrochromism for a new color electronic paper,” Sol. Energy Mater. Sol. Cells, vol. 92, no. 2, pp. 136–139, 2008, doi: 10.1016/j.solmat.2007.02.027. [187] J. E. Jang et al., “P-167: 4.5ʺ Electrochromic Display with Passive Matrix Driving,” SID Symp. Dig. Tech. Pap., vol. 39, no. 1, p. 1826, 2008, doi: 10.1889/1.3069536. [188] J. Kawahara et al., “Flexible Active Matrix Addressed Displays Manufactured by Printing and Coating Techniques,” J. Polym. Sci. Part B Polym. Phys., vol. 51, pp. 265–271, 2013, doi: 10.1002/polb.23213. [189] X. Cao et al., “Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochromic Displays Using Carbon Nanotube Thin-Film Transistors,” ACS Nano, vol. 10, no. 11, pp. 9816–9822, 2016, doi: 10.1021/acsnano.6b05368. [190] J. Tyczkowski, J. Zieliński, A. Kopa, I. Krawczyk, and B. Woźniak, “Comparison between non-equilibrium atmospheric-pressure and low-pressure plasma treatments of poly(styrene-butadienestyrene) elastomers,” Plasma Process. Polym., vol. 6, pp. 5419–5424, 2009, doi: 10.1002/ppap.200930903. [191] J. D. W. Madden, R. A. Cush, T. S. Kanigan, and I. W. Hunter, “Fast contracting polypyrrole actuators,” Synth. Met., pp. 185–192, 2000. [192] G. Zhang et al., “Versatile Interpenetrating Polymer Network Approach to Robust Stretchable Electronic Devices,” Chem. Mater., vol. 29, no. 18, pp. 7645–7652, 2017, doi: 10.1021/acs.chemmater.7b03019. [193] M. H. M. Lu, M. Hack, R. Hewitt, M. S. Weaver, and J. J. Brown, “Power consumption and temperature increase in large area active-matrix OLED displays,” IEEE/OSA J. Disp. Technol., vol. 4, no. 1, pp. 47–52, 2008, doi: 10.1109/JDT.2007.900924. 157 Appendices Magenta elastomer composite and electrochemical tests We attempted to make a stretchable composite with polyurethane similar to the PEDOT/PU composite using the magenta polymer as mentioned in Chapter 3. The magenta was easier to process than PEDOT and could be dispersed in organic solvents in its polymerized form. We dissolved it at the same concentration of magenta to PU to THF as the PEDOT/PU sample, then spin-coated as we did the PEDOT/PU onto ITO, 46% BMIM OSU/PEDOT:PSS on glass, and glass. We also attempted to transfer to PU substrate similar to the PEDOT/PU, but the film was damaged by UV-Ozone treatment of 30 minutes. Results are shown in Figure 63. The properties of the Magenta/PU films were observed under optical microscope in native and strained forms. For thin films the film appeared relatively homogeneous as in Figure 64 but for thick films there was a hole-filled structure- this could be potentially due to the rapid evaporation rate of THF preventing the settling of the homogeneous film and ratios of slower evaporating organic solvents could be tried. In observing whether this material is stretchable, we look Figure 64: Magenta/PU composite optical microscope, and strain to 100% then return to 0% Figure 63: Magenta/PU on a) ITO, b) 46% BMIMOSU/PEDOT:PSS, c) PDMS-coated hydrophobic glass, d) after UV-Ozone treatment and PU thermal + pressure transfer 158 observe the porous film at 0% strain, 100% strain, and then the restored 0% strain. There is some evident differences indicated in the 100% and return samples but no evident large cracks- this could be further studied. A composite of this form of an electrochromic thiophene-derived polymer and polyurethane could open the door intrinsically stretchable multi-colour displays. Of course, being non-conductive, it would have to be paired with a transparent stretchable conductive layer- in our case demonstrated with PEDOT:PSS. We tested the magenta film and its ability to reduce and oxidize using ITO or BMIM OSU/PEDOT:PSS as an electrode. As the L* value did not change very much, we additionally used the hue values a* and b* calculated as discussed in Chapter 4 to quantify the optical time constant, as these quantities changing with time were more visible due to the similar lightness of the magenta and the magenta and PEDOT:PSS both being coloured. We tested three different cells as in Chapter 3 with Magenta/PU on ITO vs. ITO, on BMIM OSU/PEDOT:PSS vs. ITO, and vs. BMIM OSU/PEDOT:PSS. We found we were able to produce multiple colouring states similar to other multilayered electrochromic devices depending on when the magenta and PEDOT:PSS were in their transparent or coloured states. The visual contrast is shown in Chapter 3 Figure 27, while plots are shown in Figure 65. Tested on ITO vs. ITO the magenta was in neutral form at 0 V and could oxidize to transparent at +2 V. As seen the a* and b* hue change values provide a better estimate of optical time constant, of approximately 2 s for oxidation and 3 s for reduction, and electric time constants <0.6 s. The cell resistance was not greatly affected, at 290 Ω, and no semicircle for charge transfer resistance, indicating good electrical interface between the electrolyte and Magenta/PU and ITO. For the cells containing BMIM OSU/PEDOT:PSS electrodes, data displayed in Figure 65, we also observed good electrical behaviour, but an increase in cell resistance, perhaps due to charge transfer resistance at the (PEDOT:PSS)/Magenta/PU interface. It was observed that the magenta could be oxidized at +2 V, reduced at 0 V, and then the PEDOT layer reduced as well at -2 V. The limits could be optimized to produce the best optical contrast depending on how much each material is reduced. The electrical time constant was <5 s, while the optical time constant was <10 s, not significantly different from our PEDOT cell tests, indicating that similarly performing cells could be potentially produced also with Magenta/PU layers. However, the currents were lower, potentially due to the higher resistance at the PEDOT/Magenta interface. When BMIM 159 OSU/PEDOT:PSS was used as a counter electrode we found even more possible states of coloration as shown for different voltages, with similar time constants and changes <10 s as shown in Figure 65c. More in depth analysis was not performed as here was just demonstrating the concept that a multilayered cell with BMIM OSU/PEDOT:PSS electrodes could be explored using other electrochromic thiophene-derivatives layered to produce multi-coloured displays. Figure 65: Step response different magenta cells, showing voltage applied across cell (a-c), L*a* b* values unnormalized (d-f), and electrochemical impedance plots (g-i) for: a, d, g) magenta on ITO vs. ITO, b, e, h) magenta on BMIM OSU/PEDOT:PSS (B) vs. ITO, c, f, i) magenta on B vs. B counter electrode 160 Electrode optimization As briefly reported in Chapter 4, we iterated through several versions of contacts, optimizing for mechanical and chemical stability. Due to the soft-hard interface between the PEDOT and our metal wires, a concentration of stress at the mechanical dissimilar boundary often led to cracking of the soft material breaking electrical contact as in Figure 66, with versions shown in Figure 67. 1. Initially, the electrolyte was cast using Ecoflex to mask areas for electronic contact. Ag paint was used to bond copper tape to the PEDOT film and reinforced with epoxy resin. The samples exhibited a mechanical weakness at the interface between the soft plasticized electrolyte and hard epoxy resin at which the PEDOT film fractured as in Figure 66. 2. We also tried first reinforcing with epoxy and then casting the electrolyte, which improved mechanical stability, but the copper turned green and leached into the electrolyte, indicating potential corrosion. 3. The copper was replaced by soft carbon black-SIS elastomer electrodes, which provided better chemical stability and reliable contact. It was found that reinforcing the edges to be held in the clamps with epoxy resin was effective in all cases for improving mechanical durability. Shown in the figure is a carbon black ‘bridge’ structure, with a soft carbon black electrode bridge between the unstrained edge of the sample and the strained middle section, such that any fracture that develops in the hard to soft region will be bridged by the stretchable soft electrode and Figure 67: Early iterations of contact fabrication Figure 66: Mechanical failure of contact 161 remain connected. A second version was made where the carbon black electrodes were first melted onto the SEBS and then the PEDOT:PSS was then spin-coated on top and the rest of the fabrication was similar. 4. The final version is described in Chapter 4 and involves carbon black only under the epoxy at the edges, isolating it from the electrolyte in case of parasitic reactions. 162 Custom optical spectrometer reference and uncertainties In Figure 68 we observe the difference in measuring sample reflectance with the Cary 7000 DRA-2500 including or excluding specular component (SCI/SCE) and observe only a ~1.5% offset. In Figure 69 we observe the procedure for masking the DRA 2500 integrating sphere attachment and the plots when taking the 100% R baseline and 0% R zero values. The RS50 white standard is used for the 100% R baseline and Matte Rust-oleum black spray paint coated card is used for the 0% R zero, with values compared to the reference port, which was covered in an opaque 1 mm thick TeflonTM layer. Shown are the differences in the baseline and zero using the full sample port or the masked sample port. Figure 68: Cary 7000 specular component included vs excluded (SCI vs SCE) and dR (%) comparison Figure 69: a) Mounting of sample with mask onto DRA 2500 external reflectance attachment for Cary 7000, baseline 100% and 0% reflectance for without b) and with c) small sample mask 163 In Figure 70 we observe with each subsequent measurement for a test transparent sample the optical modulation decreases maximum by ~2.7%, giving an uncertainty value for optical modulation for variation in the same sample. Figure 70: Variation between runs for sample transmission Cary 7000 (~2.7%) 164 Cell three-electrode measurements In Figure 71 we observe cyclic voltammograms for hydroquinone redox in increasingly concentrated phosphoric acid and observe the offset decreases as pH decreases. In Figure 72 we observe what happens in applying our general 1.6 V step across the cell in the three-electrode configuration. Similar to the discussion in 4.4.3.6, when the top electrode is oxidized, again we observe the voltage division is larger across the reduced electrode, and similarly in the reverse direction, and more even for the opaque sample. Though, the peaks for charging are observed to be peaking for the same electrode, but this could be due to the potentiostat slow response, as it is can sometimes take 0.1 s to respond. Further tests must be done to verify the source. Figure 71: Cyclic voltammograms (10 mV/s) of standard Ag/AgCl and pseudoreference Ag/AgCl using hydroquinone redox couple in aqueous H3PO4 Figure 72: Step voltage division across cell, with half-cell voltages vs. pseudoreference Ag/AgCl wires, and luminosity L*
Thesis/Dissertation
eng
Electrical and Computer Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Intrinsically stretchable polymer based electrochromic devices for soft electronic displays
Text